Season both sides of the chicken thighs with salt and allow them to rest for 20 minutes at room temperature.
Heat a large saute pan with 2 Tbsp vegetable oil until the oil is almost smoking.
Quickly sear both sides of the chicken thighs and remove from heat.
*Note, the chicken thighs will still be undercooked in the center at this stage.
Cut the seared chicken thighs into bite-sized chunks and set aside for later.
Use the remaining 1 cup of vegetable oil to fry the sliced white onion at 350F until golden brown.
Drain crispy onions on paper towels and season lightly with salt.
Cook 1 cup of rice in 1.5 cups of chicken stock for roughly 10 minutes, or until the rice is 75% cooked but still has a small amount of bite.
When rice is 75% cooked, remove from heat and set aside for later.
In a small pan over medium heat, combine the flour with the butter and stir until combined.
Heat the milk in a separate pot until milk reaches 180F, or slightly below a simmer.
When the milk is hot, whisk milk very slowly into the butter & flour mixture.
Season this mixture with salt, pepper, and nutmeg and continue to cook for an additional 10 minutes.
Fold the 1 cup of shredded cheddar cheese into the bechamel sauce.
Fold 1 cup of sour cream into the sauce.
Add the red pepper flakes to the sauce and taste for seasoning one final time.
Preheat oven to 375F.
Oil an oven-safe casserole dish with vegetable oil or a generous amount of butter.
Add a layer of the partially cooked rice to the bottom of the dish.
Add a layer of fried onions on top of the rice.
Add a layer of parsley on top of the fried onions.
Add chicken on top of the parsley.
Add small broccoli florets on top of the chicken.
Pour the sauce you made earlier on top of the entire mixture.
Add the grated parmesan cheese on top of the casserole.
Place aluminum foil over the top of casserole and bake in oven for 25-30 minutes, or until the mixture is bubbly and hot and the rice is fully cooked.
Increase heat to 450F and cook for an additional 5 minutes, or until the parmesan cheese is golden brown.
When ready to serve, top with remaining crispy fried onions and fresh parsley.