Place potatoes into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and transfer to a large bowl. Allow to cool to room temperature, about 30 minutes.
Run 1/2 cup spinach through a juicer according to manufacturer's instructions; add to cooled potatoes and mash until smooth.
Pulse remaining spinach in a food processor until finely chopped. Add to mashed potatoes.
Heat 1 tablespoon oil in a large skillet over medium heat. Add onion and garlic; cook and stir until fragrant, about 5 minutes. Let cool slightly, about 10 minutes. Stir into mashed potatoes.
Mix water and flaxseed meal together in a small bowl; let sit until thickened, about 5 minutes. Add to mashed potatoes with baking powder and salt; mix to combine. Form into patties. Dust both sides of patties generously with flour.
Preheat oven to 425 degrees F (220 degrees C). Line a baking sheet with parchment paper.
Heat 1 tablespoon oil in a large skillet over medium heat. Cook patties in batches until a pale crust forms, about 3 minutes per side. Repeat with remaining 1 tablespoon oil and patties. Transfer pan-fried patties to baking sheet.
Bake in the preheated oven until golden brown and cooked through, about 10 minutes.
