Italian-style stuffed Portobellos

This recipe was quite fast to make and wasn't a lot of effort. The balsamic marinade really helped mitigate any overly-mushroomy flavor and made it a delight to eat.

Mats

Mushrooms
1/4c olive oil
2 Tbsp balsamic vinegar
1 clove garlic, crushed
4 portobello mushroom caps, stems and gills removed

Filling
1 small onion, diced
1/2 tsp dried basil
1/2 tsp dried oregano
1/4 tsp pepper
1/4 tsp salt
2 cloves garlic, crushed
10 sun-dried tomatoes, halved
1/2c milk
10 oz spinach or arugula. I used a mix of the two greens
4oz mozzarella cheese, grated
6 cherry tomatoes, halved
2 Tbsp breadcrumbs, lightly toasted

Preheat your oven to 375
Spray a baking sheet with cooking oil and place the mushroom caps, gill-side up in the pan
Combine the oil, vinegar and garlic in a mixing cup and stir well to make a vinaigrette
Pour marinade over the mushroom caps and bake for 30 minutes or until caps are soft.

Meanwhile, heat about a tablespoon of olive oil in a large pan over medium-high heat. Add the diced onion, basil, oregano, pepper and salt and saute for 5 minutes or until the onion is soft.
Add the sun-dried tomatoes and crushed garlic and cook for 30 seconds, stirring often.
Add the milk and bring to a boil. Cook for 3 minutes
Add the spinach/arugula and cook until wilted. Spinach is really bulky, so you'll find it easier if you add the greens in two batches and wait for the previous one to wilt. Stir often, coating the leaves with the milk/onion mixture. It will take about 4 minutes for each batch to wilt down.
Add the mozzarella and stir well. Cook for 4 minutes, stirring constantly.

When the mushrooms are done, remove them from the oven and put them on plates. Top each mushroom cap with 1/4 of the spinach mixture, 3 cherry tomato halves and a sprinkle of the breadcrumbs.