Heat oven to 350 degrees.
Season beef short rib with salt and pepper. Heat a large dutch oven over medium-high heat. Add 2 tablespoons vegetable oil, sear beef on both sides, working in batches, and remove from pot.
Keep pot over medium heat, add in tomato paste and cook for 2 minutes. Add the onions, carrots and celery. Brown slightly. Add the stout. Reduce heat to low, reduce slightly. Add the beef back to pot. Add beef stock. Bring to a simmer, cover and braise in oven until tender, about 45 minutes.
Remove meat from braising liquid. Strain liquid into a sauce pan, bring to a boil and whisk in cornstarch to thicken slightly.
Spread tater tots out on a large baking sheet. Bake until golden brown and crispy, about 20 minutes.
Heat oven broiler to medium high. Reheat braised beef in a large saute pan with pearl onions, mushrooms and thickened braising liquid. Once hot, add in heavy cream and fold in truffle paste. Pour mix into metal dish (about an 8x8-inch pan or casserole dish), top with crispy tater tots. Sprinkle gruyere cheese over top. Bake under broiler until cheese is melted. Serve hot.