Portobello mushrooms are so meaty in texture that they are often thought of as the vegetarian answer to steak. In other words, serve this as main course or side dish. Easy as is, this can be made easier still: Substitute classic pesto, made with basil, from the jar.
If you do have time, there is a way to enhance this already lovely combination. Add a dozen or so roasted shallots to the platter.