Saute the onion in olive oil and butter on medium heat, for about 3-4 minutes.: Warmth and a soft, fragrant base are the first rewards here, as the air fills with a sweet onion aroma and the surface of the pan will show faint sizzles. You want the onion to become translucent, losing its raw edge while developing a gentle sweetness, not to brown aggressively. If the heat is too high you risk caramelizing too fast, which alters the intended mild flavor. Keep the pan at medium, and stir occasionally until the pieces are tender and glossy, that sheen is your visual cue that they are ready.
Stir in garlic and spinach and saute about another 5 minutes, until there is no liquid remaining. Add salt and pepper to taste.: As soon as the minced garlic hits the warm onion, a sharp, savory scent will bloom, so watch it closely to prevent burning. Adding the bulk of chopped spinach yields an immediate steam off the leaves, and you will notice the pile collapse into a silky mass. The key here is to cook until the pan no longer pools water, which prevents soggy filling and ensures the final texture is pleasantly firm. Season lightly with salt and freshly ground pepper , tasting for balance. A common mistake is leaving too much moisture, which makes it hard for the eggs to bind later, so be patient and let the liquid evaporate.
Transfer spinach onto the plate, and place in a cold place for a few minutes. Meanwhile, prepare the egg mixture by whisking the rest of the ingredients together.: Moving the cooked spinach to a plate stops residual heat from overcooking it, preserving a vibrant color and peppering texture. Chilling briefly helps any remaining steam condense so you can squeeze out excess liquid if needed. While it cools, whisk the eggs , add the crumbled feta cheese , grated Parmesan cheese , and chopped dill into a bowl until combined and glossy. The whisking aerates the mixture slightly, which will encourage a light set in the oven. Avoid mixing while everything is piping hot, as that will make the filling watery and prevent proper binding.
Add spinach to the egg mixture. Stir.: When the cooked spinach is cool enough, fold it into the prepared egg and cheese mixture so the temperature difference does not scramble the eggs . You will feel the texture change as the greens integrate with the creamy custard, and the aroma will mellow into a cohesive savory scent. Stir just until evenly combined, preserving a slightly chunky texture rather than puréeing it, which helps each mini cup maintain pleasant mouthfeel. A mistake to watch for is overmixing to the point where the filling becomes pasty, losing the delightful flecks of herb and cheese.
Divide the filling between the 15 Fillo shells. Bake at 350 F for 15 minutes. Let cool slightly before serving.: Spoon the prepared filling into each of the 15 Athens Mini Fillo Shells , watching that each cup receives a similar amount so they bake evenly. As they go into a 350 F oven, you will notice a warm, savory scent growing and the edges of the fillo turning golden and crisp. Baking for about 15 minutes sets the custardy interior while giving the tops a gentle color; use visual cues more than the clock, looking for bubbling at the edges and a firm set in the center. Allow the bites to rest so the filling firms slightly, making them easier to pick up. A common error is opening the oven too often, which lowers temperature and can prevent a proper set, so try to resist frequent peeks.
