This dish is hearty and stick-to-your-ribs good! But it is also VERY sophisticated, so feel free to take it into the rest of the week, or even present it to your lucky dinner guests! Sorry I didn't get a photo of this to post--we ate ours too fast to take one!
Set a large pot of salted water to boil for the pasta. Slice onions, mushrooms, and olives. Chop garlic and set aside.
Heat a large frying pan over medium heat, add oil and butter. When butter bubbles, add onions and a bit of salt to help them soften. Stir well to coat. Reduce heat to medium low. Cover partially and continue to cook, stirring often, for about 20 minutes until onions are caramelized. You can add a bit of sugar to help the process along.
When water boils, cook pasta 1 minute less than al dente according to box directions. Reserve 1/2 cup or so of pasta water. Drain and keep warm.
Add garlic to caramelized onions, then mushrooms and fennel seeds. Saute for about two minutes till mushrooms are softened. Add olives, basil, and 1/3 to 1/2 cup reserved pasta water, then add all the cooked pasta to the pot. Stir well, turn up heat, cover and bring quickly to a gentle boil.
Remove from heat and gently fold in feta cheese taking care not to break up chunks if possible. Taste and adjust salt and pepper.
Serve with a hearty Burgundy or Cabernet Sauvignon wine and crusty Italian bread.