Combine 1/2 teaspoon salt, pepper, and 3-4 Tblsp earth balance butter, melt together. Lightly coat both sides of caps with butter mix. Place on grill rack for 3 minutes on each side, then an additional 1 min per side at highest heat to crisp edges.
Combine shallots, Worcestershire sauce, mustard, and butter in a small bowl, stirring well. Cut rolls in half horizontally, spread a touch of butter on cut side, grill on cut sides til golden brown. Spread shallot mixture evenly over cut sides of rolls. Layer on 1 portobello cap (I put mine upside down on the bun so when I top with carmelized onions they stay in the slider with each bite ;))and top with carmelized onions on bottom half of each roll; top with top halves of rolls.