Brush any dirt off 1 pound mushrooms and break into pieces. Place into the bowl of a food processor, and pulse until mushrooms are finely minced.
Heat 1 Tablespoon canola oil in a large skillet over medium-high heat. Add the minced mushrooms and onions and ½ teaspoon kosher salt, and cook, stirring often until most of the moisture has been cooked out and the mushrooms begin to clump together (it will resemble cooked ground beef), about 10-15 minutes.
Heat oven to 350°F. In a large bowl, mix together the cooked mushroom mixture, 1 pound lean ground beef, ⅓ cup panko, ¼ cup reduced-sodium soy sauce, 1 large egg, 2 Tablespoons brown sugar, 1 Tablespoon grated fresh ginger, 1 Tablespoon gochujang, 4 cloves garlic, and 2 teaspoons sesame oil (you can mix with your hands or a large spoon), until uniform.
Transfer the mixture to an 8.5"x4.5" (1 pound) loaf pan, or form into a free-form loaf shape on a foil-lined baking sheet. Bake the meatloaf for 45 minutes.
While the meatloaf is cooking, stir together 2 Tablespoons gochujang, 2 Tablespoons brown sugar, 1 Tablespoon reduced-sodium soy sauce, 1 Tablespoon rice vinegar, and 1 teaspoon sesame oil in a small bowl. Set aside.
After 45 minutes, remove the meatloaf from the oven and brush the top (and the sides, if you made a free-form loaf) with some of the Gochujang glaze. Return the meatloaf to the oven for 15 more minutes, then brush a second layer of glaze over the meatloaf. Check the temperature of the meatloaf. If it's 160°F or greater, turn the oven on to broil, and return the meatloaf to the oven to brown the glaze. If it hasn't reached 160°F yet, leave the oven at 350°F and bake for 10-15 more minutes. Broil the glaze for a few minutes after the baking is done, if needed.
Remove the meatloaf from the oven and let it rest for 10 minutes before cutting. Cut the meatloaf into 8 slices and serve with the leftover glaze. Garnish with sesame seeds, if desired.