Toast the oats. Spread the rolled oats (75 g / ¾ cup) in a dry skillet over medium heat, or on a baking sheet in a 350°F oven, and toast 4-5 minutes, stirring often, until fragrant and lightly golden. Let cool completely.

Brown the butter. Melt the butter (70 g / 5 Tbsp) in a small saucepan over medium heat. Swirl occasionally as it foams, then browns - you'll see golden-brown flecks and smell a nutty aroma, about 4-5 minutes. Immediately pour into a small bowl to stop the cooking, and cool to room temperature (it should still be pourable, not solid).

Mix the dough. In a large bowl, whisk the active starter (100 g / ½ cup) into the water (300 g / 1¼ cups) until mostly dissolved. Add the bread flour (450 g / 3¾ cups) and whole wheat flour (50 g / ⅓ cup) and mix until no dry patches remain - the dough will look shaggy. Cover and rest 30 minutes.

Add the mix-ins. Sprinkle the salt (10 g / 1¾ tsp) over the dough, then add the cooled brown butter, maple syrup (42 g / 2 Tbsp), and 60 g of the toasted oats, reserving the rest for topping. Knead by hand or with a stand mixer's dough hook 6-8 minutes, until smooth, elastic, and only slightly tacky.

Bulk rise. Cover the bowl and let the dough rise at room temperature 5-6 hours, doing 2-3 sets of stretch-and-folds in the bowl during the first 2 hours. The dough is ready when it looks puffy and has grown by about half.

Shape. Turn the dough out onto a lightly floured surface and pat into a rough rectangle. Roll or fold it into a log the length of your loaf pan, and settle it seam-side down into a greased 9x5-inch loaf pan.

Cold retard. Cover the pan and refrigerate overnight, 8-12 hours - this slows the rise so you can bake on your own schedule and deepens the flavor.

Bake. The next day, preheat the oven to 375°F (190°C). Brush the top of the cold loaf with a little extra maple syrup and scatter the reserved toasted oats (15 g) over the top. Bake 40-45 minutes, until deeply golden and the internal temperature reads about 200°F (93°C).

Cool. Let the loaf cool in the pan 10 minutes, then turn it out onto a wire rack. Cool at least 1 hour before slicing, so the crumb has time to set.