– Preheat oven. Line a baking sheet with parchment, then place cleaned portobellos cap side down on the baking sheet. Drizzle with olive oil, kosher salt, and black pepper. Bake, then set aside.
Heat olive oil in a skillet over medium-high heat. Sauté onion until lightly browned and soft, stirring occasionally. Add sun dried tomatoes and garlic, then cook 1 minute more.
Add butter to pan, then melt. Add breadcrumbs to pan and cook until toasted and liquid has absorbed. Transfer filling to a large bowl.
Add parmesan cheese, tarragon, and basil, then stir to combine. Carefully spoon filling on top of portobellos, then cover with slices of fontina cheese.
Bake until cheese has melted. Drizzle with olive oil and extra herbs or parmesan cheese if wanted.