Cook chopped potatoes in boiling salted water to cover 20 minutes or until tender. Drain potatoes, and set aside.
Meanwhile, prepare Roasted Portobello Caps.
Sauté garlic in butter in a large saucepan over medium heat until butter and garlic are golden. Add milk, salt, and pepper. Bring to a simmer; remove from heat, and add potatoes. Mash with a potato masher until almost smooth.
To serve, spoon Yukon Gold Mash into Roasted Portobello Caps; top with Morel Sauce. Garnish, if desired.