Preheat oven to 350°F.
In a large saucepan, melt the butter over medium heat. Add the onions and sauté until soft, about 4 minutes. Add the garlic and cook for another 30 seconds, stirring often.
¾ cup unsalted butter, 1½ cup onion, 3 cloves garlic
Add the flour and stir constantly with a wooden spatula for about 2 minutes.
½ cup all-purpose flour
Whisk in the broth and half and half, whisking constantly to remove any lumps. Use a wooden spatula or spoon to stir until thickened, about 2 to 3 minutes.
1½ cup chicken broth, 1½ cup half and half
Stir in the cheese and continue stirring until melted.
8 oz fontina cheese
Turn off the heat and stir in the thyme, Dijon, salt, pepper, and lemon juice.
1 tsp dried thyme, 2 tbsp Dijon mustard, 2½ tsp Kosher salt, 1 tsp black pepper, 1 tsp lemon juice
Stir in the chicken, rice, and broccoli and continue stirring until fully mixed. Spray your baking dish with cooking spray and then transfer the mixture into the dish (it will be very thick).
cooking spray
In a medium-sized bowl, toss the bread crumbs with the 2 tbsp of melted butter. Sprinkle the buttered breadcrumbs over the casserole.
2 cups bread crumbs, 2 tbsp unsalted butter
Bake the casserole for 30 to 40 minutes, or until bubbly and the breadcrumbs are toasty and lightly browned.
Let rest for 10 minutes before serving.