Bruschetta of Spring Vegetables

Name: Bruschetta of Spring Vegetables
Ingredients: 6 baby artichokes (about 1 pound); 1/2 lemon; 1 cup shelled fresh fava beans (from 1 pound in the pod); 6 tablespoons extra-virgin olive oil, plus more for brushing; 8 small spring garlic cloves or 4 medium garlic cloves, thinly sliced; 1 small shallot, minced; 8 medium asparagus, peeled and cut on the diagonal into 1/2-inch lengths; 12 scallions, cut into 1-inch lengths; 1 cup shelled fresh peas (from 1 1/2 pounds in the pod); Salt; 1 tablespoon chopped flat-leaf parsley; 1/2 tablespoon finely chopped chives; 1/2 teaspoon finely chopped tarragon; Eight 1/2-inch-thick slices of sourdough bread, crusts removed