Slice onion and dice garlic finely and add to the Dutch oven with butter, on medium heat, and add a pinch of salt and dash of pepper. Sauté until onions and garlic are tender and translucent (about 5 minutes).
Zest and juice lemon. Finely chop dill.
Take Dutch oven off the heat, and add your salmon fillets on top of the onions. Add a dash of pepper and sprinkle salt over fish.
Mix dill with cream, sour cream, lemon juice, and zest. Pour over the fish.
Place the lid on Dutch oven and bake for 10-15 minutes (depends on fillet thickness) until internal temperature of fish is 120-125°F.
Gently scoop salmon out of Dutch oven into a serving dish.
Garnish with lemon slices and some reserved dill.
Notes
No need to cut the salmon in smaller fillets before cooking.