Buffalo Chicken French Bread Pizzas

Name: Buffalo Chicken French Bread Pizzas
Ingredients: 4 tbsp. salted butter; 2/3 c. Buffalo-style hot sauce (such as Frank's RedHot); 1/4 tsp. garlic powder; 1/4 tsp. onion powder; 4 c. shredded rotisserie chicken, skin removed; 1 loaf soft French bread, halved lengthwise and cut crosswise into thirds; 2 c. shredded mozzarella cheese; 1/4 small red onion, thinly sliced; Juice of 1 lemon; 1/2 tsp. kosher salt; Black pepper, to taste; Pinch of cayenne pepper; 1/4 c. olive oil; 3 celery stalks, thinly sliced, plus 2 cups celery leaves; 2 c. thinly sliced romaine lettuce; 1 carrot, grated; 3 tbsp. crumbled blue cheese