Preheat oven to 450°F. (See Notes for how to cook on a gas grill.)
Spray an oven-proof 9x13 baking dish (glass or ceramic) with olive oil spray.
Place portobellos, gill side up, in prepared dish.
Mix thyme, sage, garlic, olive oil, salt and pepper in a small bowl.
Divide mixture between portobellos, spreading it out over the gills.
Transfer to oven and roast until tender, about 8 to 10 minutes.
Remove portobellos from oven and drizzle with balsamic vinegar and juices from the dish.
Serve warm.
Notes
* To cook this on a gas grill, you want the burner under the skillet to be off. With a three burner gas grill, preheat grill to 450°F by adjusting the left and right burners to medium-high and leaving the middle burner off. Once grill reaches a consistent temperature, place dish over middle burner. Avoid opening the grill cover as you lose a lot of heat. * Don't have thyme and/or sage? Use whatever fresh herbs you have. * Aged balsamic vinegar is thicker and has a more concentrated flavor. Just a little adds great flavor. You an also use regular balsamic vinegar.