1. Heat oil in a small nonstick skillet over medium-high heat; cook peppers and garlic, stirring until soft (about 4 minutes). Transfer to a bowl.
2. Combine sour cream and horseradish sauce. Chill until ready to use.
3. Preheat broiler; coat baking sheet with cooking spray. Place egg white and panko into separate shallow bowls. Dip tomatoes into egg white and coat in panko; place on prepared baking sheet. Broil 6 inches from heat about 1 minute or until panko is golden (turning after 30 seconds). Let tomatoes cool slightly on sheet.
4. Alternate breaded and unbreaded tomatoes with 1 teaspoon pepper mixture, 1/2 teaspoon horseradish sauce, and a few arugula leaves to create 4 stacks. Top with remaining unbreaded tomato slice and an arugula leaf.