Bean and Sausage Stew

Name: Bean and Sausage Stew
Ingredients: 2 tbsp. olive oil; 1 lb. thin Irish or Italian pork sausages (about 14); 1 whole large yellow onion, cut into 6 wedges, plus 4 medium yellow onions, thinly sliced lengthwise; 1 whole large carrot, cut into 6 pieces; 1⁄2 leek, white and light green parts only, quartered crosswise; 10 oz. dried lima beans, soaked overnight; 2 sprigs thyme; 1 sprig rosemary; Kosher salt and freshly ground black pepper; 5 tbsp. unsalted butter; 6 whole garlic cloves, smashed and peeled; 1 tbsp. white wine vinegar; 1 Fresno chile, stemmed, seeded, and finely chopped; 2 tbsp. whole-grain mustard, plus more for serving; 7 oz. stale sourdough bread