Heat the olive oil in a large skillet over medium heat. After 90 seconds, when the oil is hot, add the sliced onions and pearl onions and cook for 10-12 minutes, stirring occasionally, until deeply browned and caramelized.
Add the garlic, rosemary, thyme, sage, and black pepper, and cook for 2 minutes, stirring continuously.
Pour in the mushroom stock and tamari, stir well, and bring to a simmer. Reduce the heat to medium-low and cook for 10 minutes, stirring occasionally, until the pearl onions are tender.
In a small bowl, whisk the cornstarch with the water until smooth.
Slowly pour the slurry into the gravy while stirring continuously. Cook for 3 minutes over medium heat, stirring, until thickened to desired consistency.
