Melt 2 tbsp butter in a large pot over high heat. Add chicken, sprinkle with pinch of salt and pepper. Cook 1 1/2 minutes until golden.

Turn chicken, sprinkle with salt and pepper. Cook 1 minute until golden then remove onto plate - doesn't need to be fully cooked (will cook more in oven).

Rest 5 minutes, then chop into small pieces, or shred.

Preheat oven to 160°C / 325°F.

Cook mushrooms: Melt 4 tbsp butter over high heat in the same pot. Add mushrooms and cook for 4 minutes until starting to go floppy but not yet golden.

Add onion: Add garlic and onion, cook for 3 minutes until mushrooms are golden. Add a pinch of salt and pepper halfway through.

Add flour & milk: Turn heat down to medium. Add flour. Stir for 30 seconds. Add milk and stir for 10 seconds or until it thickens (see video at 52 sec).

Add cream and broth: Add chicken stock, cream, 1 tsp salt and 1/2 tsp pepper. Stir, then let the mixture heat up, stirring every now and then so it doesn't catch on the base, and cook until the sauce thickens enough so you can draw a path on the back of the spoon (video at 1.04). Do not make it very thick - it gets thicker in next steps.

Add cheese & pasta: Turn stove off. Stir through half the cheese (will melt in oven), then add pasta and chicken. Toss to mix, taste and add more salt if needed.

Bake: Pour into 23 x 33cm / 9 x 13" pan, smooth surface. Top with remaining cheese. Bake 25 minutes until golden and bubbly. Serve immediately!