01 -
Preheat the oven to 400°F. Lightly coat a 9×13-inch baking dish with nonstick spray and set aside.
02 -
Bring a large pot of salted water to a boil. Add Brussels sprouts and cook until just fork-tender, about 4 to 5 minutes. Drain thoroughly and set aside.
03 -
In a large skillet over medium heat, melt the butter. Add diced onion and cook until softened, about 3 to 4 minutes.
04 -
Stir in the minced garlic and crushed red pepper flakes, cooking for 30 seconds while stirring continuously.
05 -
Sprinkle flour evenly over the onion mixture. Stir constantly for 1 minute until a smooth roux forms.
06 -
Slowly whisk in chicken broth. Continue whisking until the sauce thickens slightly, about 1 to 2 minutes.
07 -
Pour in the heavy whipping cream. Whisk until the mixture becomes smooth, creamy, and thick enough to coat the back of a spoon, about 2 to 3 minutes.
08 -
Season the sauce evenly with kosher salt and black pepper.
09 -
Add the blanched Brussels sprouts to the sauce, mixing to coat. Gently fold in the Gruyère and Parmesan cheeses. Transfer to the prepared baking dish. Sprinkle with crispy fried onions. Bake until golden and bubbling, about 20 to 25 minutes.
10 -
Remove from oven and scatter chopped fresh parsley over the top before serving for added color and freshness.