Put mustard in a large bowl. Slowly pour oil into mustard while continuously mixing to form a smooth sauce; stir in shallot and garlic. Add cauliflower to the mustard sauce and stir to coat. Pour coated cauliflower onto a baking sheet, scraping out any shallot or garlic stuck to the bowl and sprinkling over cauliflower.
Bake in the preheated oven, stirring 2 to 3 times, until cauliflower is tender, about 45 minutes.
