Preheat oven to 400 degrees F (200 degrees C).

Peel outer skins from garlic bulbs and cut about 1/4 inch from the top of each bulb to expose cloves; place each bulb onto a piece of aluminum foil, drizzle each with about 1/2 teaspoon olive oil, and wrap garlic bulbs in the foil. Place wrapped bulbs into a baking dish.

Bake garlic in the preheated oven until soft, about 30 minutes. Let bulbs cool.

Place lentils into a 5-quart pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until lentils are tender but not mushy, about 15 minutes; stir occasionally. Drain lentils in a fine-mesh strainer and rinse with cold water to cool; drain again.

Break garlic bulbs apart and squeeze cloves to release cooked garlic. Place garlic into a large salad bowl and add grape tomatoes, parsley, and green onions. Whisk balsamic vinegar with 1/4 cup olive oil, salt, and black pepper in a small bowl. Fold lentils into tomato mixture and pour dressing over salad; gently toss to coat. Keep refrigerated until serving time.