Silky Sweet Onion Coulis

1. Melt Butter and Cook Onions

Start by melting the butter in a large heavy frying pan over medium heat. As the butter begins to foam, add the thinly sliced onions and sprinkle them with kosher salt. Stir to coat the onions evenly. The salt will help draw out the moisture from the onions, causing them to steam and sizzle, a process that should take about 10 minutes.

2. Cover and Cook Onions

Once the onions are steaming and sizzling, cover the pan and reduce the heat to medium. Let the onions cook for 10 minutes, allowing them to become juicy and soft under the trapped steam.

3. Uncover and Continue Cooking

After this initial cook, uncover the pan, lower the heat to low, and continue cooking the onions. Stir and scrape the pan frequently for the next 1 to 1 1/4 hours. This slow cooking process will develop the onions' natural sweetness, causing them to collapse and turn a rich golden color.

4. Add Rosemary

Halfway through the cooking time, add the rosemary sprig to the pan. The rosemary will infuse a lovely fragrance as it wilts into the onions.

5. Blend Onions

When the onions are ready, transfer them to a blender.

6. Deglaze Pan and Combine with Onions

Return the pan to medium heat and add the crème fraîche and broth. Simmer the mixture, using a spatula to deglaze the pan and lift all the flavorful brown bits. Once this mixture is bubbling, pour it into the blender with the onions. Add the fresh lemon juice to balance the sweetness of the onions.

7. Blend to Smooth Purée

Blend everything until you achieve a smooth purée. Taste the coulis and season with freshly ground black pepper. If you feel the purée needs a bit more creaminess, add a little more crème fraîche and blend again until you reach the desired consistency.