Preheat oven to 450°F (230°C). Place cauliflower (florets-side up) on a foil-lined baking sheet. In small bowl, whisk together olive oil, mustard and 1/2 tsp (2 mL) of salt. Brush oil mixture on cauliflower.
Cover baking sheet and cauliflower tightly with foil. Bake in preheated oven 30 min. Uncover and continue to bake another 30 min., or until cauliflower is golden brown and can be easily pierced with a knife.
Meanwhile, melt the butter in a saucepan over medium-high heat. Whisk in flour and cook 30 sec. Gradually whisk in milk and remaining salt until smooth. Bring to a boil, reduce heat to medium-low and whisk continuously until sauce has thickened, approx. 4 min.
Remove sauce from heat and stir in cheese until melted. Stir in chives (if using). Slice cauliflower into pieces or wedges and serve with cheese sauce.