Trim the Brussels sprouts, removing any discoloured leaves. Cut them into quarters. Steam until tender (about 5 minutes). Put in a serving dish and keep warm.
Melt the butter with the oil in a small sauce pot. Whisk it in the flour and cook until golden (1 minute). Slowly whisk in the milk and heat until bubbling and thickened (about 3-5 minutes). Add the salt and 1 tbs. of the lemon juice. Taste and season with pepper, adding more lemon juice to taste.
Pour the sauce over the sprouts. Sprinkle with the lemon zest.
The lemony cream sauce can do double duty as a topping to brighten up steamed cauliflower, broccoli, or asparagus. Or try it on broiled or baked fish. Serves 5.