Warm a large skillet over medium heat then add olive oil and plant butter. Add onions and a pinch of salt and cook until soft and deep gold. Stir now and then and lower the heat if they brown too fast.
Stir in garlic and thyme and cook one minute until fragrant.
Splash in balsamic vinegar and stir to lift the fond from the pan.
Pour in vegetable broth and simmer until the liquid reduces by half.
Stir in oat cream and nutritional yeast. Simmer until the sauce turns silky and coats the spoon. Season with salt pepper and lemon juice.