Heat olive oil in a large pot over medium heat. Add onion and cook for 3-4 minutes, until softened.
Add mushrooms, basil, thyme, rosemary, salt, and pepper. Cook for 7-10 minutes, stirring occasionally, until the mushrooms release most of their liquid and begin to brown.
Add garlic and cook for 1 minute, stirring until fragrant.
Sprinkle cornstarch over the vegetables and stir well to coat. Add soy sauce and Dijon mustard and stir to combine.
Gradually pour in vegetable stock, stirring well to prevent the cornstarch from clumping. Add potatoes and bring to a boil.
Reduce heat to medium-low and simmer uncovered for 15-20 minutes, stirring occasionally, until the potatoes are tender.
Add white beans. Reduce heat to low, then gradually stir in the milk. Simmer gently, without boiling, for 8–10 minutes, until the beans are heated through and the stew has thickened.
Taste and adjust salt and pepper as needed. If the stew is too thick, add a splash of milk or vegetable stock.
Serve warm, topped with fresh parsley.