Artichoke and Shrimp Gratins

Name: Artichoke and Shrimp Gratins
Ingredients: 1/4 teaspoon saffron threads; 1/2 cup dry white wine; 5 tablespoons unsalted butter , divided; 2 large yellow onions , very thinly sliced (about 6 cups); 2 1/4 teaspoons plus a pinch of kosher salt , divided; 1 cup uncooked arborio rice; 1 (8-ounce) bottle clam juice (such as Bar Harbor); 12 ounces peeled and deveined raw large shrimp , chopped; 1/3 cup panko; 3/4 cup plus 2 tablespoons grated Parmesan cheese , divided; 1 (14.5-ounce) jar marinated quartered artichoke hearts; 1 (8-ounce) container crème fraîche; 1/4 cup chopped fresh tarragon (optional); Microgreens , for garnish (optional)