Peel and finely chop the onion. Roughly chop the mushrooms. Peel and grate or crush the garlic. Pick any thick stalks out of the watercress.
Warm 2 tsp olive oil in a deep frying pan or wok. Add the beef and stir fry over high heat for 4-5 mins till the beef is browned. Lift out the pan onto a plate.
Add the onion to the pan with a splash of water. Season with salt and pepper. Fry and stir on low heat for 5-8 mins till the onion’s soft but not browned.
Stir in the garlic. Cook and stir for 1 min. Add the mushrooms. Fry for 5 mins till the mushrooms are juicy and golden brown.
While the mushrooms cook, fill a pan with boiling water. Add a pinch of salt. Bring to the boil. Add the spaghetti. Simmer for 8 mins till the pasta is tender.
Stir the Dijon mustard into the mushrooms. Add the beef back to the pan. Stir in the cream. Gently fry over a low heat for 2 mins till everything is warmed through. Taste and adjust the seasoning.
Drain the spaghetti. Add to the beef. Toss together and divide between two warm plates. Top with watercress to serve.