Clean the mushrooms. I avoid using water for this. Instead, use a clean kitchen towel to gently rub away any dirt. If there are any stubborn dirt bits then you can dampen the kitchen towel while rubbing but don't soak the mushrooms.
Roughly chop the onion and mushrooms. I left the small brown mushrooms whole and did large chunks of onion. If you want everything more evenly distributed in your baked feta pasta then you can slice/dice the onion and mushrooms into smaller pieces.
Step 2: Prepare the baking dish
Add the mushrooms, onion, garlic, and olive oil to the pan and toss well to coat everything well. Then add the fresh thyme to the center of the dish.I threw my garlic in as whole cloves that I 'smashed' once roasted. You could also finely chop/mince the garlic - just make sure it's under olive oil so it doesn't burn in the oven.
Add the feta cheese block over the herbs - which will avoid them burning in the oven. Then sprinkle everything with a little dried oregano, salt, pepper, and a little more olive oil (over the feta).
Step 3: Bake the feta and mushrooms
Bake the dish in the oven at 400ºF/200ºC for about 25 minutes - the feta should become golden in color and the mushrooms will release their juices to create a 'sauce'.
Step 4: Prepare your pasta
As the feta and mushroom bake, prepare your pasta of choice according to their package instructions. If using zoodles, you can prepare them now too - I actually eat my zoodles raw but you can cook them if preferred.Once cooked, drain the pasta and save 1/4-1/2 cup of the starchy pasta water to add to the dish to help the mushroom sauce adhere to the pasta.The remaining salty pasta water can be frozen in ice cube trays to use in stocks and soups.
Step 5: Assemble the Feta, Spinach and Mushroom Pasta
Remove the mushroom and feta from the oven and mash up the feta, mixing it well within the dish.Add the spinach to this dish and mix it in. The residual heat will wilt the spinach into the mushroom pasta.Add the cooked pasta and pasta water to the baking dish and toss well to mix everything. You can then enjoy the feta, spinach, and mushroom pasta (or zoodles) directly in the dish or serve it up into smaller dishes.
Use high-quality feta - use a whole feta block - not crumbled feta (which can be chalky and won't create a 'sauce' the same way). You could also use french feta.
For a lighter dish - you can reduce the amount of feta in the recipe and replace it with a little light cream, vegetable stock, or extra pasta water.
Read the blog post above for the answers to several FAQs as well as suggested optional add-ins and substitutions!