Preheat oven to 375 degrees F (190 degrees C). Grease an 8×10-inch casserole dish.
Make 1/2-inch slits in pork roast using a sharp knife. Slice 2 garlic cloves; place garlic slices into the slits in pork roast.
Place mushrooms in the bottom of the prepared casserole dish. Sprinkle parsley, thyme, salt, and pepper over mushrooms; grate remaining garlic clove with a microplane grater over mushrooms.
Heat olive oil in a large skillet over medium-high heat; place pork in hot skillet and cook, turning, until lightly browned on all sides, about 5 minutes. Transfer pork to the casserole dish.
Stir white wine and chicken stock together in a cup; pour into the skillet and bring to a boil while scraping the browned bits of food off the bottom of the skillet with a wooden spoon. Pour wine mixture over pork.
Roast in the preheated oven until pork is slightly pink in the center, about 50 minutes. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C). Transfer roast to a warm serving dish; cover to keep warm.
Pour drippings with mushroom mixture into a saucepan; bring to a boil and cook until liquid is reduced by half. Stir cornstarch into water until smooth; pour into drippings and cook until thickened, about 3 minutes. Slice pork roast and serve with mushroom pan sauce.
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