1 red onion, sliced into 1/4-inch rings
2 to 3 slices whole-grain bread, cubed
1/2 pound beets (about 2 small beets)
4 tablespoons extra-virgin olive oil
1 small shallot, minced
1 tablespoon honey mustard
1/4 cup red-wine vinegar
2 tablespoons sugar
1/2 teaspoon coarse salt
1/4 teaspoon freshly ground pepper
12 ounces spinach, well washed, stems removed
4 ounces button mushrooms, stems trimmed, sliced