Stir 2 tbsp (30 mL) of the broth with cornstarch in a small bowl; set aside.
Heat a large skillet over medium-high heat. Add olive oil and garlic; sauté until fragrant, about 1 min. Add mushrooms; cook until mushrooms are lightly browned, 5 to 7 min.
Add wine to mushroom mixture; cook until almost all liquid is absorbed, about 2 min. Add remaining broth and lemon juice.
Stir in cornstarch mixture, thyme and pepper. Cook until thickened, about 1 min.
If you prefer runnier gravy, finely chop mushrooms, and add more vegetable broth to thin to desired consistency.