Stir garlic and vegetable oil together in a small bowl. Arrange cauliflower florets on a baking sheet; pour oil mixture over cauliflower. Toss to coat.
Bake cauliflower in the preheated oven until tender and lightly browned, about 30 minutes.
Melt butter in a 4-quart stockpot over medium heat; cook and stir leeks and flour in the melted butter until fragrant and well blended, 5 to 10 minutes. Add cauliflower, chicken stock, and cream; simmer until flavors have combined, 20 to 25 minutes. Stir chervil, salt, and pepper into the soup; simmer until desired thickness, 10 to 15 more minutes.
