In a large bowl, season the eggs with ¼ teaspoon of salt and whisk until they are well-beaten and smooth, but not foamy. Make sure there are no large streaks of egg white.
Use a box grater to grate the parmesan cheese, it always tastes so much better when you grate it yourself!
Use a good chef's knife or a mandoline to slice the zucchini into ¼-inch thick rounds.
Heat the olive oil in an oven-safe non-stick or cast iron skillet over medium high heat. Once the oil is shimmering, add the zucchini in batches (don't overcrowd the pan) and cook about 2 minutes on each side, until the slices are golden brown.
Transfer cooked zucchini to a paper towel and season with a small pinch of salt. Turn off the heat and carefully wipe any remaining olive oil out of the pan with a paper towel.
Return the skillet to the stovetop and place over medium low heat. Melt the butter and swirl it around to the edges of the pan. Add the eggs and once they are barely set around the edges, top with the zucchini slices, then place the frittata in the preheated oven to bake. It will cook pretty fast, so don't go anywhere!
Once the eggs are just set on top (about 5-10 minutes), remove the pan from the oven, top with parmesan cheese, and place back in the oven for about 1-2 minutes until the cheese is melted.
Allow the zucchini frittata to stand in the pan for about 5 minutes before slicing, top with fresh basil and freshly ground black pepper and serve!
Notes
Don't salt the zucchini until after it's cooked! If you salt it before or while it's cooking, it will start releasing too much water and will get soggy. We want them golden brown and holding their shape, so salt at the end!
Don't put too many eggs in too small a pan: One of the reasons this zucchini frittata recipe works so well is because we cook it in a wide, shallow pan. The eggs are in a thinner layer so they cook quickly and evenly. If the pan is too small and the egg layer is too deep, the frittata will burn on the bottom before it cooks on the top.
Make sure the oil is hot before you fry the zucchini: If the oil isn't hot, the zucchini will just start absorbing it and never get golden brown.
Don't overbake a frittata: This is the #1 tip I can give you, the eggs should be just barely set and have a silky, custard-like texture.
Make lacy, crispy, cheesy edges: I like to sprinkle the parmesan out toward the edges of the frittata so you get a little bit of a crispy, cheesy treat in each slice!
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