Roast the garlic pepper fries until crisp and browned in places, 18 to 20 minutes. How to grow portobello mushrooms. Stuffed Portobello Mushrooms Recipe High protein vegan If the stems in the middle are large, i like to shorten them using a knife to gently trim them down. How to plant portobello mushrooms . After ten days or more, the portobello mushrooms should be large enough to harvest. Portobello mushrooms are best harvested when the mushroom caps are 1.6 to 2.4 inches across in diameter. Continue to add water to the compost until new mushrooms sprout up. Portobellos reach maturity when their caps are about 4 to 6 cm in diameter. Preheat oven to 425° f. Turn the mushrooms/bag so the marinade gets evenly coated. Portobello mushrooms make great “meaty” flavorful burgers and are simple to make. Preheat a wide pan or medium sauce pot over high heat. Use paper, a cardboard box, or even plastic liners to layer the growing medium. I’m not usually a big fan o...