Preheat your oven to 425°F (220°C) and line a baking sheet with parchment paper.

Toss broccoli in olive oil, salt, and pepper. Spread on the baking sheet and roast for about 20 minutes until tender and slightly charred.

In a large pot over medium heat, sauté diced onions in olive oil until translucent (about 5 minutes). Add minced garlic; cook for an additional minute.

Add roasted broccoli and vegetable broth to the pot. Simmer for 5 minutes, then blend until smooth using an immersion blender or regular blender.

Stir in heavy cream and half of the cheddar cheese over low heat until heated through.

For croutons, place baguette slices on a baking sheet, top with remaining cheddar cheese, and bake until golden brown (5-10 minutes).

Serve soup hot, topped with cheesy croutons.