Cook the pasta according to package instructions.: The kitchen will smell faintly of wheat and steam as the pot comes to a boil, and you should hear a steady rolling simmer as the pasta cooks. Use plenty of salted water so the penne pasta absorbs seasoning while it hydrates, and stir occasionally to prevent sticking. Test for al dente a minute or two before the package time, because pasta continues to soften slightly when mixed into the hot sauce. A common mistake is overcooking, which leads to a mushy texture; keep a bowl of cold water nearby to cool a test piece if you need to check doneness without interrupting the pot heat.
Meanwhile season the diced chicken thighs with salt, pepper, paprika and garlic powder. Heat 1 tbsp olive oil in a large skillet or dutch oven over medium high heat. Cook chicken in skillet, until browned and cooked through, transfer chicken to a plate to keep warm.: As you season, the spices will cling to the surface of the chicken , promising a flavorful crust when seared. Heat 1 tbsp olive oil in a large skillet or dutch oven over medium high heat, where the oil should shimmer but not smoke. When the chicken hits the pan you will hear an immediate sizzle and see edges begin to brown; that Maillard reaction builds savory depth. Cook until browned and cooked through, then transfer the chicken to a plate to keep warm so it does not overcook in residual heat. A frequent error here is moving pieces too soon, preventing proper browning; let them form color before turning.
To the same skillet add 1 tbsp olive oil and heat over medium heat. Add the veggies and cook, stirring occasionally, until crisp tender (about 3 to 5 minutes). Remove veggies from skillet to the plate with the chicken and keep warm.: After searing the chicken , the pan holds browned bits that are full of flavor. Add the next tablespoon of oil and allow it to warm until it shimmers, which helps the vegetables start sizzling right away. Add the veggies and cook, stirring occasionally, until crisp tender about 3 to 5 minutes; you'll notice bright color and a slight snap in the asparagus , and the mushrooms will begin to darken. Remove veggies from skillet to the plate with the chicken and keep warm to preserve their texture. Avoid overcooking the vegetables here, since they will finish in the sauce and you want them to remain slightly firm.
Add the last tbsp of olive oil to the skillet along with 4 tbsp of butter, over medium heat. Use a spoon or whisk to scrap up any browned bits on the bottom of the skillet (that's extra flavor!). Add the garlic, red pepper flakes and flour, whisk until mixture is thickened and smooth. Slowly pour in the milk and chicken broth, whisking continuously until the liquid is added. Finally add in the parmesan cheese and whisk until smooth and creamy.: The butter will melt and foam gently, releasing a nutty aroma that signals flavor buildup. Use a spoon or whisk to scrape up any browned bits on the bottom of the skillet, because those fond pieces dissolve into the sauce and add complexity. Add the garlic , red pepper flakes and flour , whisking until the mixture is thickened and smooth; the flour should cook for a minute to remove raw starch taste. Slowly pour in the milk and chicken broth , whisking continuously so the liquid integrates without lumps. When the mixture begins to steam gently, finally add in the parmesan and whisk until smooth and creamy, watching for a glossy sheen. If the sauce is too thin, simmer briefly to reduce; if it gets grainy, lower the heat and whisk steadily. A frequent pitfall is adding cold liquid too quickly which can cause clumps, so steady pouring and whisking is key.
Add the chicken and veggies back to the pan as well as the cooked pasta. Stir to combine and coat evenly with sauce. Garnish with chopped parsley. Enjoy!: At this point the sauce should cling to the whisk and smell rich and cheesy, with a gentle peppery lift from the red flakes. Stir to combine and coat evenly with sauce, making sure each piece of penne pasta and chunk of chicken is glossy and covered. Heat for a minute or two so everything is unified, watching that the sauce does not boil vigorously which can separate the dairy. Taste and adjust salt and pepper if needed. Garnish with chopped parsley for a fresh contrast and serve immediately so the textures stay at their best. One thing to avoid is leaving the dish sitting in the hot pan too long, which can over soften the pasta and wilt the vegetables.
