Cook pasta according to package directions. Reserve 1 cup of pasta water, drain the pasta and set aside.

Pat the salmon fillets dry with a paper towel and season them with salt and pepper on both sides. If the skin is on, keep it; you'll remove it after cooking.

Heat a large skillet over medium-high heat. Once hot, add the oil and butter. Then add the chopped onions and cook for 3 minutes. Next, add the cherry tomatoes and cook for a couple of minutes, until they start to pop and look wrinkly. Remove them from the pan onto a plate.

Add the salmon to the skillet, skin-side up first, to get a sear on the top side, and cook for 3 minutes per side until it's golden on both sides, no longer pink (wet-looking), and flakes easily with a fork. Remove the salmon onto a plate, skin-side up. Remove the skin by pulling it back and coarsely chop the salmon into pieces.

Remove any excess oil from the pan, but leave the tidbits at the bottom. Add the cream or half and half to the pan along with ½ cup of the reserved pasta water. Scrape the bits from the bottom of the pan as you bring it to a simmer. Add the pesto and mix to incorporate into the sauce.

Add the pasta, tomatoes, onions, and salmon pieces back into the skillet and toss to coat them with the sauce. If needed, add a bit more pasta water to make the sauce more saucy.

Squeeze half a lemon over the salmon pesto pasta and toss to combine.