Preheat oven to 375 degrees.
Cut the head of cauliflower into thick slices, each approximately 3/4 to 1-inch thick.
Brush the slices with canola oil, season with salt and black pepper and place on a baking sheet.
Roast for 25-35 minutes or until softened and the stem can be easily pierced with a knife.
While the cauliflower is roasting, make the sauce.
Heat butter in a small saucepan over medium heat. Add the shallot, reduce heat to low and cook 4-5 minutes, stirring often, being careful it does not brown or burn.
Add the flour and dry mustard and cook, stirring occasionally for 1-2 minutes. Do not let it brown.
Remove from the heat and pour in the beer. Return to the heat, then add the cheese in handfuls, whisking constantly over low heat until cheese melts and is smooth, 2-3 minutes. Add Worcestershire and black pepper to taste.
Preheat the broiler. Place a rack 6 inches from the heat.
Spoon or pour the sauce over the cauliflower.
Place under the broiler and broil until bubbling and beginning to brown, 2-4 minutes. Sprinkle with chives and serve.