In a small bowl, whisk together the flour, onion powder, and garlic powder. Set aside.

Melt the butter in a large skillet over medium heat. When the butter is bubbly, whisk in the flour mixture. Whisk until smooth. Gradually whisk in the milk and bring to a simmer.

Simmer until the sauce thickens slightly, whisking constantly. Whisk in the parmesan cheese. Add the spinach and cook until the leaves wilt (this happens fast), stirring frequently (mixture will be thicker than traditional creamed spinach). Season to taste with salt and black pepper. Remove the pan from the heat.

Slice the loaf crosswise, making several slices about 1 1/2-inches apart, without cutting all the way through the bottom (keep the loaf intact). Now make a few slices in the opposite direction, creating a cross-hatch pattern. Transfer the bread to a large piece of foil.

Fill the openings of the bread with the spinach mixture. Top the loaf with the mozzarella cheese and gently push the cheese into the openings, over the spinach mixture.

Wrap the bread in the foil and bake for 10 minutes. Unwrap and bake for 3 to 5 more minutes, until the top is golden brown and the cheese melts.