Preheat the oven to 350 degrees F.
Lay the bread slices in a single layer on a large baking sheet lined with parchment paper. Brush lightly with 2 tablespoons of the olive oil. Toast in the oven until lightly golden, 15 to 20 minutes, rotating halfway through. Let cool enough to handle, then rub the bread with the cut sides of the garlic clove.
Bring a medium pot of heavily salted water to a boil. Add the fava beans and boil for 2 minutes, then remove with a skimmer and plunge into an ice bath to cool and stop the cooking (keep the water boiling to blanch the peas). Remove the fava beans from the ice bath and peel the outer skin off the beans using your fingers and discard. Blanch the peas in the boiling water for 1 minute, then plunge into the ice bath. Drain and pat dry to avoid a watery puree.
Combine the favas and peas in a bowl with the remaining olive oil and mash with a potato masher or pastry cutter. Stir in the goat cheese, mint and lemon juice. Thin with additional olive oil as needed and season with salt and pepper. Cover and refrigerate until ready to serve.
To serve, top each crostini with a spoonful of the pea and fava bean puree and top with grated lemon zest. Serve immediately.