Bring a large saucepan of salted water to the boil. Cook the pasta according to packet instructions until al dente. Drain,rinse with cold water and transfer to a 2½-litre baking dish.
Rinse the saucepan and return to medium-high heat. Add the half the butter and the onion,garlic and thyme and cook until golden,about 5 minutes,stirring regularly. Increase the heat to high,add half of the mixed mushrooms and cook for a further 5 minutes,stirring occasionally,until the mushrooms are browned and softened. Transfer the contents of the pan to a bowl.
Add the oil to the pan,then stir in the spinach for a minute until it is wilted. Remove spinach from the pan and set aside,allowing any moisture released to drain away. Add the remaining mushrooms to the pan and cook until browned,about 5 minutes.
Return the first batch of mushrooms to the pan and mix well. Reduce heat to medium,add the tomato paste and paprika and cook for 1 minute,then add the flour and cook for 2 minutes,stirring constantly. Gradually stir in the stock over medium heat until a thick sauce is formed (this takes about 5 minutes). Stir in the sour cream and wilted spinach and turn off the heat.
Preheat your grill to high. Stir one-third of the sauce through the pasta to combine,then pour over the remaining sauce to cover. Scatter very lightly with cheese and grill until the cheese is golden,about 5 minutes. Scatter with chopped parsley and serve with some green salad leaves on the side.
Adam's tip: When cooking pasta for pasta bakes,you don't need to seriously undercook it – al dente is fine. It will soften further by absorbing moisture from your sauce.