Step 1

Set an oven rack in middle position and preheat oven to 400°F.

Step 2

Melt 4 tablespoons of the butter in a medium saucepan over medium-low heat. Add the flour and stir constantly for about 5 minutes or until the mixture is a golden sandy color. Whisk in the milk and bring the liquid to a boil. Simmer, stirring constantly, for a few minutes, or until the liquid is thick enough to coat the back of a spoon. Remove from the heat and stir in the cheeses, ½ teaspoon salt and cayenne pepper. It will be quite thick. Set aside.

Step 3

In a large sauté pan, heat the remaining 2 tablespoons of butter over medium heat. Add the onions and cook until soft and translucent, about 5 minutes. Add the spinach, garlic, chopped artichokes, thyme, and ½ teaspoon salt and cook, stirring occasionally, for 5 minutes more. Transfer the vegetables to a large mixing bowl.

Step 4

Add the cheese sauce to the vegetables and fold together until well combined. Transfer the mixture to an oven-safe serving dish and bake for 20 minutes, or until hot throughout and very lightly golden on top. Serve with sliced baguette.