Preheat the oven to 190°C (375°F).
Clean the mushrooms and remove the stems. Set the mushroom caps aside and finely chop the stems.
In a skillet or large saucepan, heat the olive oil over medium heat. Add the chopped mushroom stems, onion, and garlic. Sauté until the onions are translucent and the mushroom stems are tender.
In a mixing bowl, combine the sautéed mushroom stems, onion, and garlic with the breadcrumbs, grated Parmesan cheese, parsley, dried thyme, salt, and pepper. Mix well to create a stuffing mixture.
Fill each mushroom cap with a generous amount of the stuffing mixture, pressing it in slightly.
Place the stuffed mushrooms on a baking sheet or parchment paper. Brush the tops with vegan butter or olive oil to help them brown and prevent drying.
Bake the stuffed mushrooms in the preheated oven for about 20-25 minutes or until the mushrooms are tender and the tops are golden brown.
Remove from the oven and allow them to cool slightly before serving.
These measurements should help you recreate the authentic flavors of LongHorn Steakhouse stuffed mushrooms. Enjoy your delicious appetizer or side dish!