Preheat the oven to 190C/gas 5/fan170C. Bring a large pan of salted water to the boil. Throw in the pasta, stir well and return to the boil. Cook for 6 minutes, then add the broccoli and cook for 5-6 minutes more until the pasta is just cooked. Drain well, then return to the pan.
Heat the oil in a wide pan, add the chicken pieces and fry until lightly browned. Tip in the mushrooms and stir fry for 1 minute, then stir in the tomato paste, cheese and cream. Gently simmer, stirring, until the cheese has melted to thicken the sauce. Season with salt and pepper.
Pour the sauce over the pasta, stirring gently until coated, then tip into a shallow ovenproof dish (about 1.7 litre capacity) and level the top.
Mix the spring onions, cheddar, garlic and almonds for the topping and sprinkle over the pasta. Bake for 20 minutes until golden.