Step 1In a 2-quart lidded soup pot or cast-iron casserole, heat 1 tablespoon of olive oil and cook the onions and garlic over medium-low heat until softened, 8 to 10 minutes.
Step 2When the vegetables are soft, add the dried beans, bay leaf and thyme and enough water to cover (1 1⁄2 to 2 cups). Bring to a simmer over high heat, then reduce the heat to low and cover the pot with the lid. After about 10 minutes, check the beans to make sure that there’s enough water (add more if necessary; there should be enough liquid to cover) and add three-fourths teaspoon salt. Continue cooking until tender, about 20 more minutes (this may vary). Take off the heat and set aside the beans. You should have about 3 cups of cooked beans.
Step 3While the beans are cooking, cook the bacon in a heavy-bottomed soup pot over low heat until the fat has rendered and the bacon is crispy. Remove the bacon with a slotted spoon and set aside; keep the pot on the heat.
Step 4In the same pot in which you cooked the bacon, over medium heat, add the mustard greens, a little at a time, stirring constantly so that they wilt. Add all the greens and stir to coat. Continue to cook over medium-low heat until the greens are wilted, dark green and very tender, about 8 to 10 minutes. Take off the heat and set aside.
Step 5Toast or grill the slices of bread. In a large bowl, stir together 2 tablespoons olive oil and the sherry vinegar. Add 3 cups of the cooked beans, the warm mustard greens, the bacon and the mâche. Toss the salad together until combined, then divide among four plates and top with a slice of toasted bread. Serve immediately.