This dish is great as a vegetarian main course. You could alternately use the same stuffing to fill about 20 smaller mushrooms (crimini, button) to use as an appetizer. The pecans add a subtle crunchy textural element.
Place portabella mushrooms, gills up, in a greased baking dish. Set aside.
Heat oil in a large frying pan over medium heat.
Add onions and thyme to pan and cook until onions are translucent.
Add garlic to pan and cook until softened, approximately 2 minutes. Be careful not to burn your garlic.
Add spinach and breadcrumbs to pan.
There will probably be little to no liquid in the pan at this point so you will need to add some water to moisten the breadcrumbs. Add a little at a time until the mixture is able to hold itself together nicely. If you add too much water, just keep the pan over the heat until the water has evaporated sufficiently.
Add the pecans.
Remove pan from heat and add parmesan cheese.
Salt and pepper to taste.
Divide mixture and mound over the portobello mushrooms.
Add a very light sprinkle of parmesan over the tops, merely for decoration, if you like.
Bake for 30 minutes until mushrooms are cooked through.