1 cup dried small French lentils, picked over and rinsed
Kosher salt and freshly ground pepper
2 bay leaves
1/2 small red onion
3 tablespoons sherry vinegar
3 slices white sandwich bread, crusts removed
1 tablespoon chopped fresh tarragon
4 skinless, boneless chicken breasts (6 to 8 ounces each)
2 tablespoons Dijon mustard
4 tablespoons unsalted butter, cubed
2 cups baby spinach
3 tablespoons extra-virgin olive oil