Bring a large pot of salted water to the boil, then add the penne pasta and cook for approximately 10 minutes until al dente.
Dissolve the mushroom stock cube in a bowl with a splash of the hot water. Set aside.
Heat the oil in a large frying pan over a medium-low heat, then add the onions. Sweat for 5 minutes until starting to soften without taking on colour.
Add the mushrooms and garlic, turn the heat up to medium-high and fry for 3 minutes until the mushrooms soften.
Add the miso paste, black pepper, dissolved mushroom stock cube and a splash of pasta water. Reduce the heat to low and simmer gently until the pasta is ready.
Drain the pasta, reserving a little more of the cooking water, and add the pasta to the pan.
Add the grated cheese and stir through. If it's dry, add the reserved pasta water.
Serve in bowls with extra grated cheese, if desired.