Preheat the oven to 350 degrees.
Butter a 2 quart casserole dish and set aside.
In the pan that macaroni was cooked in, add butter and Velveeta to the cooked and drained macaroni. (You may need to turn the heat on low to get it all melted).
Add the half and half, fontina, cheddar, sweet and sour sauce, sriracha and pepper relish and stir until combined. You don't have to have it all melted at this point.
Pour into the buttered casserole dish. Sprinkle with the bread crumbs, if using.
Bake for 25-30 minutes until hot and bubbly and bread crumbs are lightly browned.