Bring water to boil in a deep skillet or pot and cook until it’s almost done but still firm. Reserve ¼ cup of the pasta water then drain and set it aside.
In the same skillet, add 1 tablespoon of olive oil over medium heat then sprinkle in the bread crumbs and toast 3-5 minutes stirring occasionally until golden brown. Remove from the skillet and reserve. If necessary, wipe any remaining crumbs from the skillet.
Add another tablespoon of olive oil to the skillet over medium-high heat then add the chicken. Season with salt, pepper, garlic powder and 1 teaspoon of sage. Sear on both sides and cook until no longer pink inside then remove from the skillet.
Add the remaining tablespoon of olive oil to the skillet and the sliced brussels sprouts. Season with salt and pepper and cook over medium to medium-high heat 3-4 minutes until the brussels sprouts are tender with lightly browned pieces then remove from the skillet with the chicken.
Add the butter to the skillet over low heat then sprinkle in the flour, whisk until smooth and cook 1-2 minutes.
Add the milk and whisk until smooth. Season with salt and pepper and the remaining sage. Cook over medium heat about 5 minutes as the sauce thickens and bubbles.
Stir in the grated cheeses stirring constantly until they're melted and the sauce is smooth. Turn off the heat. Taste the sauce and adjust salt and pepper to your preferences.
Fold in the pasta then the chicken and brussels sprouts. Once combined, sprinkle the crispy breadcrumbs over the top and serve right away. Optional: serve the breadcrumbs separately at the table.