Bring a large pot of water to a boil over high heat and add in about 1 tablespoon salt.
While water is coming to a boil, melt 2 tablespoons of the butter in a large frying pan over medium heat. Add finely chopped onion and thinly sliced fennel, cook until softened about 8-10 minutes. Add more butter if needed so the veggies don't burn.
Add the heavy cream and 3 tablespoons butter, once the butter is melted add in the minced garlic. Bring mixture to a simmer, cook until it has thickened slightly, about a minute. Remove from heat. Season with salt and pepper. Set aside while pasta is cooking.
To the large pot of salted water, gently add in ravioli. Cook for just 4 minutes for fresh ravioli. Frozen ravioli may take 10-12 minutes to be cooked through.
Reserve 1/2 - 1 cup of pasta water to mix with the sauce to thin it a little. Drain pasta.
In a small bowl, mix the sauce with pasta water to thin it a little. Mix sauce with the pasta. On top of the cooked ravioli, add a sprinkling of parmesan cheese, finely chopped fresh parsley, and a little lemon zest (if desired).