Cook rice according to the package instructions. Transfer the rice to a medium bowl and mix the remaining 1 tablespoon of sesame oil and salt to taste.

Spread 4 tablespoons of sesame seeds evenly on a large plate. Press the tuna fillets into the sesame seeds, flipping to coat both sides thoroughly. In a large nonstick skillet, heat 2 tablespoons of canola oil over medium-high heat until shimmering. Add the coated tuna fillets and cook, turning once, until the sesame seeds are golden brown and crispy, about 4 to 6 minutes for rare. Transfer the tuna to a cutting board, leaving the skillet as is.

Heat 1½ teaspoons of sesame oil in the skillet over medium heat. Add 8 ounces of baby spinach and cook, stirring occasionally, until completely wilted, about 2 minutes. Add the remaining 8 ounces of baby spinach and continue cooking, stirring occasionally, until all the spinach is wilted and appears dry, about 6 to 8 minutes. Remove from heat and stir in 3 tablespoons of scallions, 1 tablespoon of sesame oil, 1½ teaspoons of tamari, 2 teaspoons of rice vinegar, and the grated ginger and garlic.

Combine ¼ cup mayonnaise, 1 teaspoon gochujang, and the remaining 1 teaspoon tamari in a small bowl.

Divide the rice evenly among 4 bowls. Top each bowl with approximately ⅓ cup of the spinach mixture. Slice the tuna into ¼-inch thick pieces, sprinkle with salt if desired, and arrange on top of the rice. Drizzle each bowl with 1 tablespoon of gochujang mayonnaise and garnish with the remaining 1 tablespoon of scallions.