Preheat oven to 350°. Heat a nonstick skillet over medium-high heat; add olive oil to pan. Place onion in pan; sauté until caramelized (20-25 minutes), stirring frequently. Cut 4 sheets phyllo dough in half. Lightly coat 1 piece with cooking spray; place on baking sheet coated with cooking spray. Place another phyllo half on top of the first; coat with cooking spray. Repeat until all sheets are stacked and coated. Bake until golden and crispy (10-12 minutes); cool. Top phyllo with blue cheese crumbles, sautéed onion, roasted bell peppers, and chopped sun-dried tomatoes. Top with baby arugula. Cut into 8 slices; serve.