How to Make It
Preheat the oven to 350°. Spread the bread crumbs on a rimmed baking sheet and bake for about 5 minutes, or until lightly toasted. Transfer to a bowl and stir in the 4 tablespoons of melted butter. Leave the oven on.
In a large bowl, whisk the eggs with the milk and season with salt and pepper. In a large cast-iron or nonstick ovenproof skillet, melt the remaining 1 tablespoon of butter in the olive oil; swirl the pan to coat the bottom and side. Add the garlic and cook over low heat, stirring occasionally, until golden, about 4 minutes. Add the spinach and stir with tongs until slightly wilted. Drain off any liquid and spread the spinach evenly in the pan and sprinkle with the lemon juice.
Pour the eggs over the spinach and cook over moderately low heat until the bottom begins to set, about 4 minutes. Sprinkle the top with the buttered bread crumbs and transfer the pan to the oven. Bake for 15 to 20 minutes, or until the frittata is just set.
In a large bowl, toss the mesclun with the vinegar and extra-virgin olive oil and season with salt and pepper. Loosen the frittata with a spatula and slide it out onto a large plate, bread crumb side up. Alternatively, cut the frittata into 8 wedges in the pan. Serve the frittata warm or at room temperature with the mesclun salad.