Preheat the oven to 350°F.
Arrange the slices of baguette on a rimmed baking sheet in a single layer. With a pastry brush, brush each toast on both sides with olive oil and season with salt and pepper. Transfer to the oven and bake until the crostini are crisp but not brown, about 10 minutes. Remove from the oven. Let cool. Crostini are best the same day they’re made.
Crumble the feta into large pieces over a bowl and add about half the olive oil, the thyme, red pepper flakes, and lemon zest. Stir gently to combine and let stand 30 minutes. (The marinated feta can be made ahead; wrap tightly and refrigerate for up to 3 days and let come to room temperature before using.)
Bring a small saucepan of salted water to a boil. Add the peas and cook until just tender, about 5 minutes. Drain and transfer to the bowl of a food processor. Add the remaining olive oil, mint, salt, and lemon juice and pulse into a chunky mash.
Spread some of the pea mash on each crostini and top each with some of the marinated feta and freshly ground black pepper.