Pour warm water into a small bowl and sprinkle yeast over the top; let sit until a bubbly foam forms on top, about 5 minutes.

In a large bowl, whisk together milk, 2 tablespoons of the butter, honey, salt and egg. Add yeast mixture and whisk to combine.

Add all-purpose flour, whole wheat flour and flaxseed and stir until a shaggy dough forms.

Turn out onto a floured surface and knead until smooth and elastic, about 5 minutes. Lightly oil a large bowl and place dough inside.

Cover loosely and let sit in a warm spot until dough is doubled in size, about 1 1/2 hours.

Lightly butter a 9x13-inch baking pan. Punch down dough and, on a floured surface, roll out into a 12x18-inch rectangle.

Brush all over with 2 tablespoons of the butter, cut into 3 strips, each 4-inches wide, and then cut each strip into 8 rectangles.

Loosely roll each piece of dough, buttered side in, and arrange in the pan seam-side down, making 3 rows of 8.

Brush with remaining 2 tablespoons butter and scatter oats over the top. Cover loosely and refrigerate until puffed and risen, 3 to 4 hours.

Let rolls sit, covered, in a warm spot for 2 hours, and then preheat the oven to 350°F.

Uncover and bake until rolls are golden brown, about 25 minutes. Serve warm.