Heat oven to 350℉. Coat a baking sheet with nonstick spray. 

Toss mushrooms with 1½ teaspoons oil and ½ teaspoon salt on baking sheet; arrange in a single layer on two-thirds of the sheet. Roast mushrooms until liquid starts to release, 10 minutes. Add chickpeas to remaining third of the sheet and continue to roast until mushrooms have halved in size and chickpeas look dry and cracked, 20 minutes more. 

Meanwhile, pulse onion, celery and garlic together in a food processor until minced, about 10 pulses. Heat 1½ teaspoons oil in a large nonstick skillet over medium-high heat, add onion mixture and cook, stirring occasionally, until vegetables are deep golden brown, about 12 minutes; transfer back to food processor and wipe skillet clean. 

Add roasted mushrooms and chickpeas, sorghum, miso, Worcestershire, paprika, chili powder and remaining ½ teaspoon salt to food processor with onion mixture; pulse mixture until a paste forms, about 20 pulses. Add sunflower seeds and pulse just to combine. 

Form veggie burger mixture into six ½-inch-thick patties (4 ounces each), pressing firmly to compact the mixture. 

Heat 1½ teaspoons oil in now empty nonstick skillet over medium heat until shimmering, add half of the patties, and cook until sides are brown and crispy, 4 minutes per side. Transfer burgers to a plate and let rest 5 minutes before assembling. 

Repeat with remaining 1½ teaspoons oil and veggie patties.