1. Cook the Onion
Begin by heating the tablespoon of melted butter in a large saucepan or soup pot over medium heat.
Add the chopped onion and cook gently, stirring occasionally. Continue until the onion becomes soft and translucent. This usually takes several minutes.
Cooking the onion before adding the other ingredients helps bring out its natural sweetness and creates a flavorful foundation for the soup.
2. Prepare the Butter and Flour Mixture
Once the onion is tender, add the remaining 1/4 cup of butter to the pot. Allow it to melt completely.
Sprinkle in the flour and stir continuously. The flour combines with the butter to form a smooth mixture known as a roux.
Cook the mixture briefly while stirring. Avoid letting it become overly dark because the goal is to create a light, creamy base rather than a deeply browned sauce.
3. Add the Half-and-Half
Gradually pour the half-and-half into the pot while whisking or stirring constantly. Adding it slowly helps prevent lumps from forming.
Continue stirring as the mixture warms. The flour will gradually thicken the liquid, turning it into a creamy base.
Keep the heat moderate rather than allowing the mixture to boil aggressively. Gentle heating helps maintain a smooth texture.