1. Preheat the oven to 435°F (225°C).

2. Carefully cut the cauliflower and broccoli into small florets. Mince the garlic.

3. In a large bowl, toss the vegetables with olive oil, salt, and black pepper until evenly coated. Spread the mixture on a parchment-lined baking sheet.

4. Roast in the preheated oven for 15 minutes or until lightly browned and tender.

5. While the vegetables roast, peel and dice the potatoes.

6. In a large pot, combine the roasted vegetables, diced potatoes, water, and vegetable broth cubes. Bring the mixture to a boil over high heat.

7. Reduce heat to maintain a simmer. Cook uncovered for 15 minutes, or until all ingredients are tender.

8. Use an immersion blender to partially puree the soup, leaving some chunks of vegetables for texture. Alternatively, blend the soup in batches using a traditional blender.

9. Stir in the frozen chopped spinach and heavy cream. Season with additional salt and pepper to taste.

10. Allow the soup to cool slightly before dividing it into airtight containers or freezer bags. Freeze for up to 3 months.

11. To serve, reheat the soup in a saucepan over medium heat until warmed through. Ladle into bowls and top each serving with crumbled feta cheese and a sprinkle of fresh dill. Enjoy!