Slice tomatoes in fairly thick slices, if you are using a smaller size portabello, you will need on one slice per portabello.
Drizzle olive oil on both sides of portabello and tomato slices. Sprinkle with cracked black pepper if desired.
Place on med heat grill and cook on both sides until tender. The tomatoes will cook quicker than the mushroom.
Remove and place portabello on plate, top with a little cheese and grilled tomato. If necessary you can pop into micro for a few seconds to melt cheese, or place several on a baking sheet and place in hot oven or low broiler just long enough to melt cheese.
Top with a healthy dallop of Thai chile paste and some fresh chopped cilantro. Salt to taste.
FYI I did not drizzle with balsamic vinegar but I bet that would add an awesome layer of flavor.