Put a non-stick pan onto a low-medium heat, add the onions and olive oil and stir to combine. Cover with lid and allow onion to 'sweat' for 10 minutes till soft - don't let it brown.
In a jug make up the gravy granules with the boiling water, stir well making sure there are no lumps - best to use a whisk for this. Add to pan, along with mustard. Stir with wooden spoon to combine well. Very gently simmer for 5 minutes. Remove from heat, stir in cream, pepper, and thyme if using.
Best not to use a strong mustard, such as English or French, for this. We've found that the sweet hamburger type works best (or grainy Dijon), especially if you're cooking for kids.