1. Preheat your oven to 200°C (400°F).

2. In a large skillet, cook the diced bacon over medium heat until crispy. Remove the skillet from the heat and set aside.

3. Using the same skillet, add the sliced onion and cook until caramelized, about 5 minutes. Next, add the flour and cook for another minute.

4. In a large bowl, whisk together the heavy cream, chicken broth, salt, and pepper. Add the cooked onion mixture and stir everything together.

5. In a greased 23x33 cm (9x13-inch) baking dish, arrange the chicken breasts in a single layer. Pour the cream mixture over the chicken, followed by the grated aged cheddar cheese and crispy bacon.

6. Sprinkle the chopped basil over the top of the dish and cover with aluminum foil. Bake in the preheated oven for 20 minutes, then remove the foil and continue baking for an additional 10-15 minutes, or until the chicken is cooked through.

7. Serve the dish hot, accompanied by roasted potatoes and steamed asparagus if desired.