2 tablespoons unsalted butter (1/4 stick)
1 cup coarsely chopped yellow onion
1/4 cup dry sherry
2 medium garlic cloves, thinly sliced
4 fresh thyme sprigs, leaves removed
2 pounds white mushrooms, sliced (about 12 cups)
4 cups mushroom broth or water
3/4 cup heavy cream
2 teaspoons freshly squeezed lemon juice
10 to 15 thin baguette slices
3 ounces blue cheese, crumbled (about 2/3 cup)