In a medium pan combine beer, thyme, garlic and red pepper flakes. Place over medium heat and bring to a simmer. Place brats in pan and simmer for 5-7 minutes until cooked through. Shut off heat and allow brats to cool in liquid. Remove and refrigerate until ready to grill and serve sandwiches (this can be done ahead of time). Strain and reserve poaching liquid.
In a small pot add cheese sauce and bring to a simmer. Add 1 T of reserved beer braising liquid, mix in mustard and shredded cheddar. Stir until cheese has fully melted. Keep warm.
Bring a grill to medium-high heat and wipe down grates with oil to create a non-stick surface. Brown brats all over – about 2-3 minutes. Grill sliced peppers and onions until charred. Season with salt and pepper.
Using a long bread knife, carefully slice the entire loaf open to create a sheet of connected slider tops and sheet of connected slider bottoms. Brush both cut sides with melted garlic butter.
Place on grill and mark off both sides of bread, warming them through.
To assemble, slice brats on the bias into bite sized pieces. Spread out on bottom sheet of bread, Pour over cheese sauce. Top with grilled peppers and onions and shower liberally with grated parmesan. Place top sheet of bread on and serve.