Preheat the oven to 375 degrees F (190 degrees C).
Arrange cauliflower florets on a baking sheet. Stir together oil and garlic in a small bowl; pour over cauliflower and toss to coat.
Bake 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 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 pepper, chervil, and salt into soup; continue simmering until desired thickness is reached, 10 to 15 more minutes.