Preheat the oven to 400°F and position a rack in the upper third of the oven.
Slice the French bread in half horizontally so you have two long halves, and place them cut-side up on a baking sheet.
Spread the softened butter evenly over the cut side of each bread half.
Bake the buttered bread halves for 5–6 minutes until slightly crisped to help prevent sogginess from the toppings; remove from the oven.
Evenly spread 1 cup BBQ sauce over the cut side of both bread halves.
Top with the cooked chicken, then scatter the chopped red onion and sliced jalapeño (if using) over the chicken.
Sprinkle the shredded mozzarella and shredded smoked gouda evenly over the toppings.
Bake the assembled bread pizzas for about 12 minutes, until the cheeses are melted and the toppings are hot.
If desired, switch the oven to broil and broil for a few minutes until the cheese is bubbly and slightly browned—watch carefully to avoid burning.
Remove from the oven, sprinkle chopped cilantro on top (if using), slice to serving pieces, and serve warm.