Place multigrain cereal in a large bowl or the bowl of a stand mixer fitted with a kneading hook. Pour boiling water over, then let stand until mixture cools to between 105°F. and 115°F. for active dry yeast or to about 120F for Instant yeast, about 20 minutes. *Don't rush it. If your mixture is too hot, it may kill your yeast. A thermometer is handy here to make sure of the temperature before adding the yeast.
Sprinkle yeast over cereal mixture. Add 1 cup flour, the oil, brown sugar and salt and stir until smooth. Gradually mix in enough remaining flour to form a smooth, moist dough. Cover bowl with plastic wrap and let rest 15 minutes.
Turn out dough onto floured surface. Knead until smooth and elastic, adding more flour if it's sticky, about 5 minutes. **If using add-ins, scatter over the dough and knead into the dough now. **
Oil a large bowl. Add dough to bowl, then flip over the ball of dough so the oiled side is up. Cover bowl with plastic wrap and let rise in warm spot until doubled in size, about 1 hour.
Prepare the Seed Mixture by stirring together all the seeds in a small bowl. Set aside.
When dough has doubled in size, gently deflate dough by pressing down the dough, then turn out onto lightly floured surface.
If making into loaves: grease one 9x5-inch loaf pan or two 8x4-inch loaf pans. Sprinkle a couple of teaspoons of the seed mixture into the pan (or divide between two pans). For 8x4-inch loaves, divide dough ball into two equal sized pieces. For 9x5-inch loaf, leave as one piece. For both, roll out into a rectangle - about 10x20-inches for a 9x5 loaf, or two 9x18-inch rectangles fortwo 8x4 loaves. Roll up jelly rolls style, starting with the shortest side, then pinch seams together. Place rolls seam side down into prepared loaf pan(s).
If making a freeform loaf: Line a baking sheet with parchment paper. Shape loaf into a round or oval shape. Scatter 1/2 of the seed mixture onto the baking sheet (about the size of the loaf) and place loaf on top.
If making a round skillet-baked loaf: Grease a 10/12-inch cast iron skillet well. Sprinkle 1/2 of the seed mixture into the skillet. Form dough into a ball and place into skillet.
For all loaves, cover with a clean kitchen towel and let rise until doubled in size, about 30 minutes.
Preheat oven to 425F.
Before baking, brush top of loaves with water and sprinkle with remaining seed mixture. *If you find the seeds don't stick well after baking, you can brush top with 1 egg mixed with 1 tsp water instead OR with 1 whisked egg white.
Bake in preheated 425F oven until golden and crusty, about 25-30 minutes for the 8x4-inch loaves/round skillet loaf and up to 45 minutes for the 9x5-inch or freeform loaf. CHECK LOAF after 20 minutes and cover loosely with a sheet of aluminum foil if it's already browned enough.
You can replace up to 1 cup of the unbleached all purpose flour with whole wheat flour, if you like. Resulting bread will be a little less light. Add-Ins are optional, but if using, you can use any large seed (such as pumpkin, sunflower etc), nut (pecans, walnuts etc.) and/or dried fruit. Chop if necessary and keep to the same approximate amount as the recipe suggests (about 2/3 cup total).