Preheat your oven's broiler.
In a heavy 8-inch nonstick, oven-safe skillet, heat the olive oil over medium-high heat.
Add the mushrooms and cook until they begin to brown, about 3 minutes.
Reduce the heat to medium.
Add a generous pinch of salt and the minced garlic, stirring until fragrant, about 30 seconds.
Stir in the blanched and chopped broccoli raab.
Cook for another minute to heat through and combine the flavors.
Season with salt and pepper to taste, then remove the skillet from the heat.
In a medium bowl, whisk together the 4 eggs, 1 tablespoon (15 ml / 0.5 fl oz) of milk, and ¼ teaspoon of salt until just combined and slightly frothy.
Stir in the grated Parmesan cheese.
Pour the egg mixture over the vegetables in the skillet, gently shaking the pan to distribute everything evenly.
Return the skillet to medium-low heat and cook, without stirring, until the edges are set, about 4 to 5 minutes. The center will still be slightly wet.
Transfer the skillet to the oven and broil for 2 to 3 minutes, or until the frittata is puffed, golden brown, and the center is fully set.
Keep a close eye on it to prevent burning.
Let it cool for a few minutes before slicing and serving.
Don't over-whisk the eggs. You want them just combined with some streaks of white and yolk remaining, which helps create a more tender texture.
