Combine the butter and garlic in a saucepan over medium heat; heat together until the butter melts completely. Add the cream into the butter mixture and simmer 3 to 5 minutes, stirring constantly. Melt 1 3/4 cup Parmesan cheese into the cream mixture. Stir in the onion and remove from heat.
Heat the olive oil in a large skillet over medium heat; cook the mushrooms in the oil until they begin to brown. Stir in the hearts of palm, Mexican-style corn, arugula, and green beans; add to the cream mixture; season with salt; transfer to a large baking dish; top with 3/4 cup Parmesan cheese.
Bake on top rack in the preheated oven until the cheese has browned, 40 to 50 minutes.