Preheat oven to 350 degrees F. Coat a 9-by-5-inch loaf pan with cooking spray.
Combine sunflower seeds, pumpkin seeds, flaxseed and sesame seeds in a dry medium skillet; toast over medium heat, stirring, until lightly brown and starting to pop, 5 to 7 minutes.
Reserve 2 tablespoons of the seed mixture in a small bowl; transfer the remaining seeds to a large bowl. Add flour, baking powder, baking soda and salt to the large bowl; whisk to combine. Whisk eggs in a medium bowl, then stir in buttermilk, oats, oil and honey (or maple syrup). Pour the wet ingredients into the dry ingredients; stir and fold together until combined. Scrape the batter into the prepared pan. Sprinkle with the reserved seeds.
Bake until golden brown and a toothpick inserted into the center of the loaf comes out clean, 45 to 55 minutes. Let cool in the pan for about 30 minutes before turning out onto a wire rack to cool completely.