Preheat oven to 350 degrees Fahrenheit.
On a lined baking sheet, add the sliced bell peppers and onion. Roast until the edges are charred. About 30 minutes depending on the side of the slices.
To make the sauce, combine the roasted red peppers, roasted onion, oat milk, salt, pepper, and garlic powder in a blender or food processor and blend until smooth.
In a 9x9 pan, combine the hearts of palm rice, sundried tomatoes, spinach, and sauce.
Cover and bake for 30 minutes, or until heated through.
Serve hot.