Bring a large saucepan of water to a boil. Stir in wild rice, reduce to a simmer, and cook (uncovered) until tender, about 40 minutes. Drain rice.
Meanwhile, melt 2 tablespoons butter in a large saute pan over medium heat. Toast pecans, stirring frequently, until fragrant, about 5 minutes. With a slotted spoon, transfer pecans to a plate, and season with salt.
Add remaining 1 tablespoon butter, the pearl onions, and 1/2 cup water to pan; simmer until water is evaporated and onions are tender and pale golden, about 10 minutes.
Trim brussels sprouts, then score an X into the bottom of each one. Place in a steaming basket or colander set in a saucepan filled with 1 inch of water. Bring water to a boil, then reduce to a simmer; cover and steam until just tender enough to pierce with the tip of a sharp knife, 6 to 8 minutes.
In a bowl, whisk together sherry vinegar, maple syrup, and mustard. Add rice, brussels sprouts, and onions; toss to coat, and season with salt and pepper. Coarsely chop pecans and add just before serving, warm or at room temperature.