Preheat the oven to 350 degrees F.
Using a serrated knife, trip the upper quarter off the length of the top of the loaf of french bread. Hollow out the center of the loaf, leaving the shell. Set aside.
Melt the butter in a large skillet on medium high heat. Add the chopped onion and cook for 2 minutes. Sprinkle in the salt and pepper and add the spinach. Cook for an additional 2 minutes, stirring occasionally until there isn't much water from the spinach in the pan.
Add in the chopped red peppers and stir for 2-3 minutes.
Stir in the yogurt, cream cheese and cheddar cheese. Reduce the heat and cook, stirring constantly until the cheeses melt.
Remove pan from the heat.
Fill the hollowed out bread loaf with the spinach-cheese mixture and top with the additional cheddar cheese.
Wrap the loaf in aluminum foil.
Place the loaf on a baking sheet and bake for 1 hour.
Remove from the oven and unwrap.
Serve warm with chips for dipping, if desired, or cut the bread into 2-inch slices.