Heat grill on medium. In a large baking dish, combine oil, garlic, chipotle, and salt. Add asparagus, shiitakes, and green onions; toss to coat. Grill asparagus until tender and lightly charred, turning occasionally; 5 to 6 minutes. Grill shiitakes and green onions until lightly charred, turning occasionally; 4 to 5 minutes. Transfer vegetables to cutting board.