Place 4 tablespoons sesame seeds on a large plate. Place tuna fillets in the sesame seeds; flip to coat both sides. Heat 2 tablespoons canola oil in a large nonstick skillet over medium-high heat until shimmering. Add the tuna; cook, flipping once, until the sesame seeds are golden brown and crispy, 4 to 6 minutes for rare. Transfer the tuna to a cutting board. Do not wipe the skillet clean.

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

In a small bowl, combine ¼ cup mayonnaise, 1 teaspoon gochujang and the remaining 1 teaspoon tamari.

Prepare 2 (8.8-ounce) packages rice according to package directions. Transfer the rice to a medium bowl; stir in ¼ teaspoon salt and the remaining 1 tablespoon sesame oil. Divide the rice among 4 bowls. Top each with about ⅓ cup spinach mixture. Slice the tuna ¼-inch thick, sprinkle with the remaining ¼ teaspoon salt and arrange on top of the rice. Drizzle each bowl with 1 tablespoon gochujang mayonnaise; sprinkle the bowls with the remaining 1 tablespoon scallions.