Prepare Morel Sauce.
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.