Preheat oven to 400 F. Option to line a rimmed baking sheet with parchment paper for easy clean up.
Toss cauliflower and yellow onions in oil and spread in an even layer on the baking sheet. Sprinkle with 1 teaspoon of salt and 1/2 teaspoon of pepper and bake in preheated oven until the veggies are tender, 20-30 minutes (depending on how large your florets are).
In a large pot over medium heat, add the butter. Once melted, add the garlic and saute until fragrant, 2 minutes. Quickly add the vegetable broth, rosemary, roasted cauliflower, and onions.
Bring soup to a boil, then reduce to a simmer and place the lid on top. Simmer for 15 minutes
Remove from heat and transfer soup to a blender, along with the lemon juice, additional salt, and pepper--I always add 1 teaspoon of salt and 1/2 teaspoon of pepper here. Blend until completely smooth.
Ladle into bowls, top with additional minced rosemary or parsley, and enjoy!