PREPARE the dressing by adding 2 cloves minced garlic, 1 tsp Dijon mustard, and 1/4 cup balsamic vinegar to a small bowl. Whisk together to combine.
DRIZZLE in olive oil, while continuing to whisk, until emulsified. Season with 1/2 tsp kosher salt and 1/4 tsp red pepper flakes. Stir and set aside.
ADD chopped hearts of romaine and radicchio to a large bowl. Sprinkle with 1/2 tsp kosher salt and toss with your hands.
SPRINKLE lettuces with diced red onion, roasted red pepper, avocado, garbanzo beans, and sliced hearts of palm. Use tongs to gently toss and evenly combine.
POUR about half of the prepared dressing around the edges of the bowl. Toss to evenly coat. Add more dressing, as desired.
SERVE immediately. Store leftover dressing in an air-tight container in the refrigerator for up to five days.