Cut the onion into fine cubes. Cut the red and green bell peppers into narrow strips. Cut the tomatoes into eighths. Mince the garlic clove.
Heat the butter in a large pan over medium-high heat. Add the onion cubes and fry for 2-3 minutes until softened. Add the minced garlic and cook for 1 minute. Add the tomato pieces and stew for 5 minutes, stirring occasionally. Stir in the tomato paste and whipped cream. Season with a pinch of sugar, salt, pepper, and sage. Bring to a boil (100°C / 212°F), then reduce heat to low and simmer for 2-3 minutes. Taste and adjust seasoning as needed. Keep warm on low heat.
Heat a portion of butter in a separate pan over medium-high heat. Add the pepper strips, season with salt and pepper, and sauté for 4 minutes until tender-crisp. Transfer to the warm sauce.
Cut the liver into thin strips. Heat remaining butter in a pan over medium-high heat. Add the liver strips and fry for 4 to 5 minutes, turning occasionally, until cooked through (internal temperature 71°C / 160°F). Season with salt and pepper. Add to the sauce and stir gently.
Ladle the liver and pepper stew into bowls and serve over rice.