Prepare your vegetables: cut the mushrooms in slices (about 3mm thick - they will shrink down), cut the spring onions in 1cm chunks, finely dice the onion, peel and press the garlic.
In a pan, add a bit of olive oil and sauté the mushrooms until brown and caramelised. Just a minute before they're done, toss in the spring onions and shake the pan up. Put everything in a bowl and set aside for later.
In the same pan, add a new bit of olive oil if needed, and sauté the onion and garlic until translucent. Deglaze the pan with the white wine. Cook on medium for about four minutes, so the alcohol evaporates.
If eating with pasta, now's a good time to start boiling some water.
Add the water and the stock cube, stir well so it dissolves. Also add the soy sauce, onion powder, turmeric, thyme and dill. Put the heat on high until the water boils, then turn down to low and simmer for about 10 minutes.
Add the cream and the mustard to the sauce and stir.
If you'd like to thicken up the sauce, stir some cornstarch and some water together in a separate little bowl, and pour the mixture into the sauce while stirring.
For extra indulgence, optionally add a tiny little bit of rich, creamery butter to the sauce and make it extra.
When the pasta is almost ready, start finishing up. Squeeze some lemon juice into the sauce to freshen it up, and stir through the prepared mushrooms and spring onions. Taste for seasoning, but I find that it's salty enough with the stock cube and the soy sauce.