Preheat the oven to 180 degrees C or 350 degrees F. On medium heat, sauté onions and garlic in a pan until the onions soften. Add spinach, stir briefly to heat, then set aside.
In a separate pot, melt butter over high heat. Remove from heat, add flour, and stir to create a paste. Return the pot to high heat, add one cup of milk, whisk vigorously to break down the paste and thicken the milk. Repeat with the second cup of milk, stirring continuously for 2 minutes.
Add grated cheese, garlic, and diced basil, stirring for a few more minutes as the sauce thickens. Introduce the spinach/onion/garlic mixture, season with salt and pepper, and stir until well combined. Remove from heat.
Pour the creamy filling into the hollowed-out bread bowl. Place the bread chunks and the lid around the bread bowl on the baking tray. Bake at 180 degrees C/350 degrees F for 10 minutes. Once out of the oven, gently stir the cheese-spinach sauce, and serve immediately. Enjoy the gooey goodness!