Avocado Tuna Sandwich

Name: Avocado Tuna Sandwich
Ingredients: 2 tbsp. fresh lemon juice; 4 tsp. French’s Classic Yellow Mustard; 1/4 tsp. ground cumin; 1/4 tsp. garlic powder; 1/4 tsp. red pepper flakes; 1/4 c. extra-virgin olive oil; Kosher salt; Freshly ground black pepper; 3/4 c. english cucumber, finely chopped; 1/4 c. red onion, finely diced; 1/4 c. yellow, orange, or red bell pepper, diced; 1/4 c. celery, diced; 1/4 c. fresh cilantro leaves, chopped; 2 large avocados, peeled, seeded, and diced; 2 (5-oz.) cans yellowfin tuna, drained, preferably oil-packed; 8 slices whole wheat or other preferred bread