Bring a large pot of salted water to a boil and cook the pasta shells according to package directions until al dente. Drain and set aside.

Meanwhile, heat the olive oil in a large skillet over medium heat. Add the chopped onion and sauté for about 5 minutes, stirring occasionally, until it softens and develops a slight golden hue.

Add the ground beef to the skillet, breaking it apart with a spoon as it cooks. Stir occasionally and let it brown evenly, about 5 minutes. If excess fat accumulates, carefully drain most of it from the pan.

Stir in the minced garlic and chili powder, allowing the flavors to meld for about 30 seconds. Pour in the tomato sauce and beef broth, stirring well to combine. Reduce the heat to a gentle simmer and let the sauce cook for 4-5 minutes, allowing it to thicken slightly while ensuring it remains saucy.

Slowly stir in the heavy cream, blending it into the sauce until smooth. Season with salt and pepper to taste. Let the cream heat through for a minute, then add the drained pasta shells to the skillet, tossing to coat them evenly in the rich, flavorful sauce.

Serve immediately while warm, garnishing with fresh herbs or grated cheese if desired.