Make the Dough
Pour the water into a small bowl and sprinkle the yeast over it. Set aside. Combine the oats and sugar in a very large bowl. Pour in the hot milk and stir to dissolve the sugar. Set aside for 5 minutes.
Whisk the eggs and yolks to liquefy them, then add to the oat mixture. Cut the butter into pieces and add it to the oat mixture, along with the lemon zest, vanilla, salt, and dissolved yeast. Stir 3½ cups of the flour into the liquid, then beat vigorously with a wooden spoon for 100 strokes. Set the dough aside for 10 minutes.
Gradually add enough of the remaining flour to make a soft, kneadable dough. Turn the dough out and knead it, or knead it right in the bowl if bowl is big enough, for 5 to 6 minutes, using just enough flour to keep the dough from sticking. Place the dough into a lightly oiled bowl and cover with plastic wrap. Set aside in a warm, draft-free spot until doubled in bulk, 1-½ to 2 hours.
Prepare the Pans
The best choice is either two 10-inch round, heavy aluminum cake pans, 2-½ to 3 inches deep, or one deep 13x9-inch casserole. (If you use the one larger pan, you will put everything into just that one pan, naturally.)
Melt the butter, brushing some of it onto the sides of the pans. Divide the rest of the butter evenly between the two pans, tilting to spread it around. Sprinkle each pan evenly with ½ cup of the brown sugar. Set aside.
Make the Filling
Put the brown sugar and cinnamon into a food processor and run the machine for a few seconds. Add the raisins and nuts and process again until finely chopped. Put the melted butter near your work area.
Assemble and Bake:
Flour your work area and turn the dough out onto it without punching the dough down. Pat or roll the dough into a 12x18-inch rectangle, with the long side facing you. Brush with the melted butter, then spread the filling evenly over the dough, pushing it down lightly with your hands. Starting at the long side nearest you, roll the dough up snugly, like a carpet.
Using a dough scraper or sharp knife, cut the dough into 12 equal slices and lay them into the pan(s) equally spaced, spirals up. (In each of the round pans, put 1 in the center and 5 around it; in the rectangular pan, make 3 rows of 4.) Press the spirals down to spread them slightly. Cover with plastic and set aside in a warm spot until almost doubled in bulk. Preheat oven to 375 degrees F about 15 minutes before baking time.
When the buns have risen to double their original size, bake for 30 to 35 minutes, until golden brown (make sure to check color after 30 minutes). If you like, spoon a hot mixture of ½ cup maple syrup or honey, 2 tablespoons butter, and 1 tablespoon lemon juice over the buns 5 minutes before they come out (half over each batch if using two pans). As soon as you take them out of the oven, invert them onto a baking sheet, taking care not to let the hot syrup spill on you. Let cool 10 minutes or so before serving.