Pour the remaining two tablespoons of olive oil into the pan. Add the potatoes and season with salt and pepper. Toss well. Turn the heat down to medium, cover, and allow to cook for 5 minutes. Remove the lid and add the squash, pepper, and onion. Turn the heat back up to medium-high and saute until the potatoes are tender and the vegetables are browned (4 to 7 minutes). Fold the mushrooms back into the pan of vegetables. Add in the Hollandaise sauce and mix.