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Sweet Potato and Avocado Sandwich

1 small sweet potato, peeled and cut into 3 slices about 2 inches thick

1 Tbsp. plus 2 tsp. mustard

1 Tbsp. soy mayonnaise

4 slices whole wheat bread

4 red onion slices

1/2 small avocado, peeled, pitted, and sliced

4 tomato slices

Salt and pepper, to taste

1/4 cup shredded Monterey Jack cheese

1/2 cup finely shredded lettuce