Stir in the starch, Dijon mustard, crème fraîche and Worcestershire sauce until well combined. Then slowly add the broth; stirring the pan constantly. Add the thyme leaves and season with salt and pepper, to taste.
Simmer for a 2-3 minutes and then stir in the white beans, broccoli and peas.
Note: If you prefer a thicker sauce, then sprinkle an additional 1 teaspoon of arrowroot powder (or other starch) and stir well to combine.
Transfer the filling to a casserole (I use a 32 x 22 cm / 2litre form). Alternatively, you can also use a large pie dish.