Roast pepper over flame until skin is charred, let cool, peel and julienne (can use jarred roasted red peppers). Drizzle Portabella mushroom with olive oil and grill until tender, remove from grill and slice; set aside. Sauté onion in butter with minced garlic and fresh herbs until tender. Toss in spinach leaves and roasted red peppers and sauté a minute or two longer until spinach is wilted.
Halve focaccia roll and assemble with Portabella mushroom slices topped with spinach sauté, then lay provolone on sandwich and allow to melt under broiler; top with sliced tomato, halve and serve warm.