Turn your Instant Pot to the 'Sauté' mode. Add the butter and olive oil, allowing it to melt.
Add the sliced onions to the pot, stirring to coat them with the butter and oil. Cook for about 10 minutes, stirring frequently until the onions start to caramelize.
Stir in the garlic, sugar, salt, black pepper, and thyme. Cook for an additional 2-3 minutes.
Pour in the beef broth and white wine, scraping the bottom of the pot to loosen any browned bits.
Close the lid, and set the valve to 'Sealing.' Cook on high pressure for 20 minutes.
Once cooking time is up, carefully release the pressure. Open the lid and stir the soup.
Preheat your broiler. Ladle the soup into broiler-safe bowls, topping each with a slice of French bread and a generous portion of Gruyère cheese.
Place the bowls under the broiler for about 2-3 minutes, until the cheese is bubbly and golden brown. Keep a close eye on it to prevent burning.
Remove from the oven and let cool for a minute before serving.