In the bowl of a food processor, combine the scallions, carrots, bell pepper, and cilantro and pulse until the vegetables are finely chopped, 2 or 3 pulses. Transfer the veggies to a large bowl.
Put the beans into the food processor and pulse until finely chopped, 2 or 3 pulses. Add the black beans to the bowl.
Put the oats into the food processor and pulse until finely ground. Transfer the oats to the bowl and add the garlic powder, cumin, chili powder, cayenne, salt, ketchup, and egg. Mix well and form into 4 equal patties.
In a large nonstick skillet set over medium-high heat, heat 1½ teaspoons of the oil. Add 2 of the patties and cook until crisp on one side, about 4 minutes. Carefully flip and cook until the second side is crisp, about 4 more minutes. Transfer to a warm plate. Add the remaining 1½ teaspoons of oil to the skillet and repeat the cooking process with the remaining 2 patties.
To serve, put the hamburger buns on a clean work surface, place a patty on each of the 4 bottom halves, and layer each with a romaine leaf, a fourth of the tomato slices, a few red onion rings, and finally the top halves of the buns.
The cooked, cooled patties will keep in an airtight container in the refrigerator for up to 5 days. To freeze, put the patties on a parchment paper–lined baking sheet and freeze until completely firm, about 3 hours. Transfer the frozen patties to a large resealable plastic bag and freeze for up to 3 months.