Preheat oven to 375°. In a large 12 inch oven safe skillet over medium-high heat, cook bacon until crispy, around 6-8 minutes. Remove using a slotted spoon to paper towels to drain. Reserve 3 Tbsp drippings in pan, freeze or discard the rest. Drain bacon on a paper towel-lined plate, then chop.
Return skillet to medium-high heat adding 2 Tbsp butter to bacon drippings. Add onion cooking for 2-3 minutes, then add garlic. Cook for 1 minute.
To the pan add chicken stock, brussels sprouts salt, pepper, red pepper flakes and thyme. Lower heat to medium, then cook uncovered for 10-15minutes, stirring occasionally, until fork tender. (Chicken stock should evaporate.)
Remove from heat and drizzle with heavy cream, then sprinkle evenly with Gruyere and bacon.
In a small bowl, melt reserved 2 Tbsp butter. Toss with panko and Parmesan, sprinkle on top.
Place into the oven and bake until cheese is bubbly around 15-20 minutes. Broil for 1-2 minutes to brown top further, if desired.
Let stand 5 minutes, then serve garnished with additional thyme.