Spring Greens and Leek Gratin

Name: Spring Greens and Leek Gratin
Ingredients: 3 Tbsp. unsalted butter; 2 large leeks (about 8 oz.), white and light green parts only, thinly sliced; 1 bunch ramps, bulbs thinly sliced, greens coarsely chopped (or 3 large garlic cloves, thinly sliced); 2 large bunches mature arugula, coarsely chopped; 1 bunch mature spinach, coarsely chopped; 2 tsp. ground coriander; 1 tsp. kosher salt; Pinch of cayenne pepper; Finely grated zest of 1/2 lemon; 1 1/2 cups heavy cream; 1/2 cup plus 2 Tbsp. finely grated Parmesan; 3/4 loaf crusty bread, crusts removed, torn into irregular 1/2 pieces (about 3 cups torn bread); 2 Tbsp. extra-virgin olive oil