Preheat oven to 400 degrees F (200 degrees C). Spray a roasting pan with cooking spray.

Spread tomato paste onto beef bones and place in the prepared roasting pan.

Bake in the preheated oven until bones begin to brown, about 30 minutes.

Transfer bones to a slow cooker and pour in enough water to cover bones. Add onions, carrots, garlic, and bay leaves to broth mixture.

Cook on Low for at least 24 hours.

Strain broth through a fine-mesh strainer into a container and refrigerate.