Sauté Aromatics: Heat the olive oil in a large pot or Dutch oven over medium heat. Add the diced yellow onion and cook until it softens and becomes translucent, about 5-7 minutes. Stir in the minced garlic and cook for an additional minute until fragrant, being careful not to burn it.
Build the Roux: Sprinkle the all-purpose flour over the cooked onions and garlic. Stir continuously for 1-2 minutes to create a roux, allowing the flour to cook out its raw taste. This step is crucial for thickening your soup and preventing a grainy texture later on.
Add Liquids & Vegetables: Gradually whisk in the vegetable broth until the mixture is smooth and lump-free. Bring the liquid to a gentle simmer. Add the chopped broccoli florets and grated carrot (if using). Cook for 10-15 minutes, or until the broccoli is tender-crisp. This is how to make broccoli cheddar soup thicker naturally, by softening the vegetables.
Incorporate Milk & Seasonings: Reduce the heat to low. Slowly whisk in the milk, dry mustard powder, ground nutmeg, black pepper, and salt. Heat gently, stirring often, but do not allow the soup to boil once the milk is added, as this can cause separation.
Melt the Cheese: Remove the pot from the heat. Gradually add the freshly shredded sharp cheddar cheese, stirring constantly until it is completely melted and smoothly incorporated into the soup. Adding cheese off the heat prevents it from clumping or turning grainy, which is often why broccoli cheddar soup can become grainy.
Blend for Smoothness (Optional): For an extra smooth consistency, use an immersion blender to blend a portion of the soup directly in the pot until you reach your desired texture. This also helps to thicken the soup further without adding more flour.
Serve Hot: Taste and adjust any seasonings as needed. Serve your healthy broccoli cheddar soup hot, garnished with a sprinkle of extra cheddar or fresh herbs, and enjoy!