Heat a heavy sauce pan over medium heat and add the butter and bacon. Let the bacon cook until the fat renders (melts). The bacon should almost be cooked but not browned. Then add the onion, garlic and red pepper and gently cook, turning the heat down to medium-low to avoid any browning. Cook until the onion and garlic begin to soften and release their flavors. Then season with the paprika and a pinch of cayenne pepper for extra heat, if desired.
Next, sprinkle the mixture with the flour and stir to form a roux. Let this cook gently for about 2 minutes, being careful not to let the bottom scorch or burn. Then add the beer and stir to combine. Add the milk a bit at a time to temper it. This will prevent lumps from forming. Continue until all of the milk has been incorporated. Finally, add the thyme.
Turn the heat up to medium, if needed, to bring the sauce up to a gentle simmer. Once the mixture starts to bubble, reduce the heat to low. Stir occasionally and let cook for about 10 minutes or so. While the sauce cooks, go ahead and cook the pasta.
You now have a flavored variation of a classic béchamel sauce.