In a small bowl, combine rolled oats, sunflower seeds, flax seeds, and sesame seeds. Pour 1 cup of warm water over the mixture, stir well, and let it sit for at least 30 minutes, or while you prepare the yeast, to soften the grains and seeds.

In a large mixing bowl (or the bowl of a stand mixer), combine 1 1/2 cups warm water and active dry yeast. Stir gently and let it sit for 5-10 minutes until foamy, indicating the yeast is active.

Add honey, salt, and olive oil to the activated yeast mixture. Stir until well combined.

Add the whole wheat flour and the soaked multigrain mixture (including any remaining water) to the wet ingredients. Mix briefly until just combined.

Gradually add the bread flour, about 1/2 cup at a time, mixing until a shaggy dough forms. Turn the dough out onto a lightly floured surface or attach the dough hook to your stand mixer. Knead for 8-10 minutes by hand or 6-8 minutes with a stand mixer on medium-low speed, until the dough is smooth, elastic, and passes the windowpane test.

Lightly grease the large mixing bowl. Place the dough in the bowl, turning once to coat. Cover the bowl with a clean kitchen towel or plastic wrap and let it rise in a warm, draft-free place for 1.5 to 2 hours, or until doubled in size.

Gently punch down the risen dough to release the air. Turn the dough out onto a lightly floured surface and gently flatten it into a rectangle approximately 9x12 inches. Roll the dough tightly into a log, pinching the seam closed at the bottom. Tuck the ends under slightly to create a neat loaf shape.

Lightly grease a 9x5 inch loaf pan. Place the shaped dough seam-side down into the prepared loaf pan.

Cover the loaf pan with a clean kitchen towel and let it rise in a warm, draft-free place for another 45-60 minutes, or until the dough has nearly doubled in size and extends about an inch above the rim of the pan.

While the bread is doing its second rise, preheat your oven to 375°F (190°C). If desired, gently brush the top of the risen loaf with the beaten egg wash and sprinkle with the extra mixed seeds.

Place the loaf pan in the preheated oven. Bake for 30-35 minutes, or until the crust is golden brown and the internal temperature reaches 200-210°F (93-99°C) when measured with an instant-read thermometer. If the top browns too quickly, you can tent it loosely with aluminum foil for the last 10-15 minutes.

Remove the bread from the oven and immediately turn it out onto a wire cooling rack. Let it cool completely for at least 1-2 hours before slicing. Slicing too early can result in a gummy texture.