Slow-Cooker French Onion Chicken

sliced (about ⅓-inch wide) yellow onions (about 1 pound)

melted butter

Worcestershire sauce

rounded teaspoon Dijon mustard

all-purpose flour

sugar

fresh thyme leaves or ¼ teaspoon dried

sea or kosher salt and fresh black pepper

boneless chicken breasts

+ ⅓ cups shredded Swiss cheese or 3 - 4 thick slices