Preheat the oven to 400ยบ F
In a large bowl, toss broccoli florets in 2 Tbsp of olive oil to coat evenly. Salt generously, and spread them out evenly on a baking sheet. Roast in the oven for about 18-20 minutes, or until tender and slightly crispy around the edges. Remove from the oven and set aside.
In a pan over medium heat, add the remaining 1 Tbsp of olive oil. Once hot, add the onions and allow them to cook until slightly translucent, stirring frequently. Add the garlic and reduce the heat to medium low. Stir contstantly until very soft and fragrant (about 2-3 minutes), then remove the pan from the heat.
Add the onion mixture and 2/3 of the broccoli to the a high powered blender. With the remaining broccoli, shave off the tops so you're left with very small florets for garnish/texture, and add the remaining stems to the blender. Add the stock and cumin, and blend for about 3-4 minutes or until very smooth. If you're not able to fit all 4 cups of stock into the blender, just add as much as you can and stir in the rest afterwards.
Pour the mixture into a large pot, and place over medium heat. Bring to a simmer, then stir in the butter, milk, and lemon juice. Remove from heat, season to taste, then cover and let stand for about 20 minutes to allow the flavours to marry.
Either stir in the remaining broccoli florets, or serve as garnish on the soup with some drizzled cream or olive oil. Enjoy!