Bring a large pot of salted water to a boil. Cook the penne according to the package instructions until al dente. Drain and set aside, reserving some pasta water.

While the pasta is cooking, heat olive oil and butter in a large pan over medium heat. Add the onion and garlic, sautéing until soft and translucent, about 3-5 minutes.

If using white wine, pour it into the pan now, allowing it to simmer and reduce by half, about 2 minutes.

Lower the heat and add the heavy cream, stirring to combine. Let it simmer gently for a few minutes to thicken slightly. Season with salt and pepper to taste.

Stir in the smoked salmon strips and chopped fennel or dill. Cook for an additional 2-3 minutes, just until the salmon is heated through. Be careful not to let it boil vigorously to avoid curdling the cream.

Add the drained penne to the sauce, tossing to ensure the pasta is evenly coated. If the sauce is too thick, add a little reserved pasta water to reach your desired consistency.

Serve the pasta hot, garnished with extra fennel fronds or dill.