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adjust rack to top position and preheat oven to 425 degrees. toss potatoes on a baking sheet with a large drizzle of oil, garlic powder, onion powder, paprika, salt, and pepper. roast on top rack until browned and tender, 20-25 minutes.
season tomato with salt and pepper.
place chickpeas in a medium bowl and mash with a potato masher or fork until about half the chickpeas are smooth (leave some larger pieces for texture). add scallion greens, parsley, guacamole, garlic powder, a large drizzle of olive oil, and as much lemon juice as you like. season generously with salt and pepper. stir until combined.
brush each side of sourdough slices with a drizzle of olive oil. heat a large pan over medium-high heat. add bread; toast until golden brown, 2-3 minutes per side
top half the toasted sourdough with chickpea salad and seasoned tomato. close sandwiches and halve on a diagonal. divide sandwiches and potato wedges between plates. Serve.