1/2 cup ricotta cheese, drained of excess liquid
Finely grated zest of 1/2 lemon
1/8 teaspoon fine salt, plus more for seasoning
Freshly ground black pepper
2 tablespoons unsalted butter, divided
2 thin slices prosciutto (about 1 ounce), cut into 1 1/2-inch pieces
1 cup fresh or frozen peas
2 tablespoons water
1 teaspoon finely chopped fresh mint leaves
1/2 teaspoon freshly squeezed lemon juice
2 (1-inch-thick) slices brioche
Place the ricotta, lemon zest, and 1/8 teaspoon salt in a small bowl, season with pepper, and mix to combine; set aside to come to room temperature.
Melt 1 tablespoon of the butter in a large frying pan over medium heat. Add the prosciutto in a single layer and cook, flipping occasionally, until crisp and golden-brown, about 5 minutes. Remove with tongs to a paper towel-lined plate and leave the rendered fat in the pan. While the prosciutto is cooking, make the peas.
Melt the remaining 1 tablespoon butter in a small saucepan over medium heat. Add the peas and water, season with salt and pepper, and simmer, stirring occasionally, until the peas are tender, 3 to 5 minutes. Remove from the heat and smash with a potato masher or bottom of a Mason jar until most of the peas are broken up (they will absorb the excess liquid). Stir in the mint and lemon juice. Taste and season with salt as needed; set aside.
Place the frying pan back over medium heat. Add the bread slices, swipe them across the bottom of the pan on both sides to absorb the grease, and toast until golden-brown, 1 to 2 minutes per side. Transfer the brioche to serving plates.
Divide the ricotta over the brioche and spread into an even layer. Dollop the pea mixture over the ricotta and gently spread into an even layer. Using your hands, break up the crispy prosciutto into small pieces and sprinkle over the peas. Serve immediately.