Heat grill to medium heat. In medium bowl, toss together plums, onions, 1 tablespoon olive oil and 1 teaspoon Adobo. Add plums and onions to hot grill grates. Grill plums, flipping once, until charred and just tender, about 4 minutes. Grill onions, flipping once, until crisp tender, about 8 minutes. Transfer plums and onions to cutting board until cool enough to handle. Halve plum wedges lengthwise. Trim root ends of onions to separate into strips; set aside.
In large bowl, whisk together vinegar, chopped chipotle peppers, adobo sauce, honey, remaining 3 tablespoons olive oil and remaining ½ teaspoon Adobo. Toss in arugula, plums and onions until coated with dressing. Top with Manchego cheese and hazelnuts.
To make this Arugula Salad with Grilled Plums and Manchego into a main course, simply top with grilled pork tenderloin or shrimp.