Preheat oven to 400 degrees. In a small pot, bring 1 cup water to a boil with the stock concentrate and a large pinch of salt. Once boiling, add the Basmati rice, cover and simmer for 15 minutes, until tender.

Meanwhile, quarter the tomatoes. Strip the thyme leaves off the sprig, then roughly chop. Thinly slice the basil leaves. Roughly chop the sun-dried tomatoes.

Toss the tomatoes on a baking sheet with 1/2 tablespoon olive oil, half the thyme, and a pinch of salt and pepper. On the other half of the baking sheet, rub the Portobello caps with another 1 tablespoon olive oil and season with salt and pepper. Roast the tomatoes and Portobellos for 10 minutes, until softened. Set the roasted Portobellos aside, then roughly chop the roasted tomatoes once they are cool enough to handle.

Meanwhile, heat 1/2 tablespoon olive oil in a large pan over medium heat. Add the sun-dried tomatoes and cook, tossing, for about 1-2 minutes, until softened. Add the spinach and cook, tossing another 3-4 minutes, adding a splash of water, if necessary, until wilted. Season with salt and pepper.

Remove pan from heat and stir in the sour cream, chopped tomatoes, remaining thyme, half the panko, and the basil. Taste and season with salt and pepper.

Stuff the spinach mixture into the Portobello caps and sprinkle with remaining panko and the Parmesan. Return the stuffed Portobellos to the baking sheet and bake for another 10 minutes, until golden brown on top.

Serve the Portobello caps on a bed of Basmati rice and enjoy!