Place the brussels sprouts, onions and carrots in a steamer basket; place in Dutch oven over 1 in. of water. Bring to a boil; cover and steam for 8-10 minutes or until crisp-tender. Transfer to a large bowl.
In a microwave, melt butter and syrup; stir until smooth. Stir in the mustard, thyme, salt and pepper. Pour over vegetables and toss to coat. Sprinkle with walnuts.