Make the dough: In a small saucepan, melt the butter over low heat. Once melted, remove from the heat and stir in the milk so it is just warm.
In a large bowl, combine the melted butter and milk with the maple syrup, warm water, and yeast and mix with a wooden spoon or dough whisk until just combined. In a separate bowl, stir together whole wheat flour, all purpose flour, oats and kosher salt. Add the flours and oats to the bowl and stir with the spoon until just combined. Turn the dough out onto a clean, lightly floured countertop and form the dough into a ball.
Knead the dough: Knead the dough by pushing with the base of your palm, then reforming it into a ball. Continue kneading for 8 minutes until the dough feels pillowy and has a smooth, stretchy exterior. If the dough is very sticky, add a small amount of flour while kneading. Alternatively: attach the dough hook to a stand mixer and start the mixer on medium-low speed, then allow the mixer to knead for 8 minutes.
Proof 1: Place the dough ball in a clean bowl and cover with a clean dish towel. Allow the dough to stand in a warm place (proof) until it rises to double in size, about 45 minutes to an 1 hour.
Shape the dough: Once proofed, grease an 8 to 9-inch loaf pan. Turn the dough onto a counter and gently press the dough into a large rectangle about 1/2-inch thick. The short side of the rectangle should be about the width of the long edge of the loaf pan. Roll the dough into a log (the width of the loaf pan). Pinch the seams on the sides and bottom of the roll and then place it into the greased pan seam-side down. Gently press the dough to fill the bottom of the pan.
Proof 2: Cover with a clean dish towel and allow to proof for 40 to 50 minutes until the dough doubles in size.
Meanwhile, preheat the oven to 350°F.
Bake the bread: Once proofed, brush with the top of the loaf with water. If desired, sprinkle the seeds on the top. Bake 30 to 35 minutes. The bread is done when the top is golden brown and the inside of the bread reaches 190°F on an instant read thermometer. Remove the bread to a cooling on a rack and cool completely, at least 90 minutes. (However tempting, cutting the bread while warm will ruin its texture!)
Serve (+ storage info): Slice the bread and serve. Store the bread at room temperature for 2 to 3 days wrapped in plastic, or refrigerator for up to a week. The bread can also be frozen, sliced into pieces and wrapped in plastic, for 3 months.