Take a large skillet with a lid.
Pour in 2 tbsp olive oil and butter and heat on a medium flame.
Toss the onions and half of the fennel in the heated olive butter mix.
Cover and simmer over low heat for 10-15 minutes.
Then add the balsamic vinegar, sugar and wine.
Turn up heat to medium high and caramelize the onions and fennel until they are browned.
Set aside and let it cool.
In a pot, heat remaining tablespoon of olive oil and saute the shallots in it.
Then add garlic and sherry and cook for about 1 minute more.
Add mushrooms, tomatoes and the rest of the fennel.
Simmer for about 3 minutes.
Pour in the stock, add basil and cook to reduce liquid by about half.
This takes about 15-20 minutes of simmering.
Add the spinach and remove from heat.
Now add the caramelized onion and fennel mix and serve hot.