Heat oil and butter in a large nonstick skillet over medium heat. Add leek to pan; cook 2 minutes or until tender, stirring occasionally. Stir in sage; cook 30 seconds, stirring occasionally. Add mushrooms; cook 10 minutes or until liquid evaporates, stirring occasionally. Add half-and-half; cook until liquid is reduced to 6 tablespoons (about 2 minutes). Stir in salt and pepper. Remove from heat; keep warm.
Cook pasta according to package directions, omitting salt and fat. Drain. Place in a large bowl. Add mushroom mixture to pasta; toss gently to coat.