Whisk together extra-virgin olive oil, balsamic vinegar, minced garlic cloves, light mayo, salt and pepper.
Place baby spinach and cannellini beans in a large serving bowl; set aside.
Heat grill to medium-high. Slice 1 large yellow onion across equator into 1/2-inch-thick slices and secure with toothpicks. Grill 5 minutes per side. Meanwhile, clean 4 portobello mushroom caps and place each in the center of a 12-inch square of aluminum foil; brush caps with dressing and fold foil over caps. Place foil packets on grill; cook 10 minutes.
Remove onions and mushrooms from grill (pouring any liquid inside packets into dressing) and chop; add to serving bowl. Drizzle with dressing and toss well to combine. Sprinkle with crumbled blue cheese; serve.