Roast Beef & Onion Grilled Cheese Sandwich

Behold, one of the best grilled cheese sandwiches I've ever tasted ~

Roast Beef & Onion Grilled Cheese Sandwich

1 Tbsp. olive oil
1 small onion (sliced)
1/4 tsp. EACH salt & pepper
1 tsp. thyme leaves
4 slices rye bread  ***or your favorite bread
4 Tbsp. whole-grain mustard
8 oz. Gruyere cheese, coarsely grated
8 slices thinly sliced roast beef

Heat 1 Tbsp. olive oil in a medium skillet over medium heat.  Add 1 small sliced onion, season with the salt and pepper and cook, covered, stirring occasionally, for 12 minutes. 

Reduce the heat to medium-low, stir in 1 tsp. thyme leaves and cook, uncovered, stirring occasionally, until the onions are golden brown, 15 to 20 minutes more (add 1 Tbsp. water to the skillet if the onions start sticking). 

Butter or oil both sides of 4 slices rye (or your favorite) bread.  Form sandwiches with the bread, spreading 1 Tbsp. whole-grain mustard, 2 oz. Gruyere cheese and 2 slices roast beef on each. 

Heat a large nonstick skillet over low heat.  Cook the sandwiches until the bread is golden brown and crisp and the cheese has melted, 4 to 5 minutes per side.

Makes 4 sandwiches