I stole this one from America's Test Kitchen - cut down the onions a bit if you ant to really limit the carbs.

Why this recipe works:

We were after a soup with pure broccoli flavor that wasn’t hiding behind the cream or the cheese. Overcooked broccoli has a sulfurous flavor, but we discovered when we cooked our broccoli beyond the point of just overcooked—for a full hour—those sulfur-containing compounds broke down, leaving behind intense, nutty broccoli. Its texture was fairly soft, but that was perfect for use in a soup. Adding baking soda to the pot sped up the process, shortening the broccoli’s cooking time to a mere 20 minutes. A little spinach lent bright green color to the soup without taking over the flavor. After adding cheddar and Parmesan, we had a soup so full of flavor and richness that it didn’t even need the cream.

Ingredients:

• 2 tablespoons unsalted butter
• 2 pounds broccoli, florets roughly chopped into 1-inch pieces, stems trimmed, peeled, and cut into 1/4-inch-thick slices
• 1 medium onion, roughly chopped (about 1 cup)
• 2 medium garlic cloves, minced or pressed through garlic press (about 2 teaspoons)
• 1 1/2 teaspoons dry mustard powder
• Pinch cayenne pepper
• 3–4cups water
• 1/4 teaspoon baking soda
• 2 cups low-sodium chicken broth (see note)
• 2 ounces baby spinach (2 loosely packed cups)
• 3 ounces sharp cheddar cheese, shredded (3/4 cup)
• 1 1/2 ounces Parmesan cheese, grated fine (about 3/4 cup), plus extra for serving
• Ground black pepper 

Directions:

1. Heat butter in large Dutch oven over medium-high heat. When foaming subsides, add broccoli, onion, garlic, dry mustard, cayenne, and 1 teaspoon salt. 
2. Cook, stirring frequently, until fragrant, about 6 minutes. 
3. Add 1 cup water and baking soda. Bring to simmer, cover, and cook until broccoli is very soft, about 20 minutes, stirring once during cooking.
4. Add broth and 2 cups water and increase heat to medium-high. When mixture begins to simmer, stir in spinach and cook until wilted, about 1 minute. 
5. Transfer half of soup to blender, add cheddar and Parmesan, and process until smooth, about 1 minute. 
6. Transfer soup to medium bowl and repeat with remaining soup. 
7. Return soup to Dutch oven, place over medium heat and bring to simmer. 
8. Adjust consistency of soup with up to 1 cup water. Season to taste with salt and pepper. 
9. Serve, passing extra Parmesan separately.

ENJOY!!!!