Meanwhile, in a large sauce pot, melt 4 tablespoons of butter over medium heat. Stir in onions and sauté until onions are soft, about 4 minutes. Add the garlic and continue to sauté for 2 minutes more, stirring constantly. Whisk in flour and cook 3 minutes, whisking constantly. Add the reserved beer, half-and-half and bring just to a boil, whisking constantly. Reduce heat and simmer, whisking often until sauce slightly thickens, about 2 minutes. Reduce heat to low. Add 12 oz White Irish Cheddar cheese and 8 oz Medium Cheddar and stir until melted and smooth. Stir in the salt, pepper and cayenne. Remove pan from heat and gently stir in the rotini and tomatoes. Pour the pasta mixture into prepared baking dish.
Melt 2 tablespoons butter and toss with the panko until well coated. Evenly distribute the panko mixture over the pasta. Bake for 25-30 minutes, or until the cheese is bubbly and the topping golden brown.