Place spelt in a medium bowl. Cover with hot water to 1 inch above spelt. Cover and let stand 1 hour. Drain and rinse with cold water; drain.
Heat oil in a medium saucepan over medium heat. Add onion to pan; cook 3 minutes or until tender, stirring occasionally. Stir in spelt, thyme, salt, and pepper; cook 1 minute. Stir in broth; bring to a simmer. Cover, reduce heat, and cook 50 minutes or until spelt is tender. Uncover and simmer 15 minutes or until liquid is almost absorbed. Remove from heat; keep warm.
Melt butter in a large nonstick skillet over medium-high heat. Add chopped mushrooms to pan; sauté 3 minutes or until tender and beginning to brown. Stir mushroom mixture into spelt mixture.