Pour the water into a small bowl and sprinkle with yeast. Set aside. Combine the oats and sugar in a very large bowl. In a small pan over medium heat, warm the milk until it shimmers, then pour it over the oats and sugar. Stir, then set aside for 5 minutes. Whisk the eggs and yolks in a separate bowl. Add the eggs to the milk, along with the butter, lemon zest, vanilla, salt, and dissolved yeast. Stir in 3-½ cups of the flour and beat vigorously with a wooden spoon for 100 strokes. Set the dough aside for 10 minutes.
Gradually stir in enough of the remaining flour, about ⅓ cup at a time, to make a soft dough. Turn the dough out onto a lightly floured surface and, with lightly floured hands, knead for 6 to 7 minutes. Dust the work surface with flour, as needed, to keep the dough from sticking. Put a tablespoon of oil into a large bowl. Using your fingers, spread it to coat the bowl. Place the dough in the bowl and rotate to coat the entire surface. Cover with plastic wrap and let the dough rise in a warm, draft-free spot for 1-½ to 2 hours, until doubled in bulk.
Melt the ½ cup of butter. Brush the bottom and sides of two 10-inch cake pans (preferably deep ones) or one 13x9-inch baking pan with the melted butter. Sprinkle cup of brown sugar in each cake pan. (If you are using a larger pan, sprinkle 1 cup of brown sugar into it.) Set aside. In a food processor, combine the remaining ½ cup of brown sugar, cinnamon, raisins, and nuts. Process until the mixture is finely chopped. Melt the remaining 3 tablespoons of butter in a small bowl.
Lightly flour the work surface and turn the dough out onto it (do not punch down the dough). Roll the dough into a rectangle approximately 18x12 inches, with the long edge facing you. Brush the surface of the dough with the melted butter. Spread the filling evenly over the dough and tamp lightly to embed. Starting with the long edge nearest you, roll the dough up snugly, like a carpet. Using a sharp, serrated knife, cut the dough roll into 12 equal slices. (To better gauge equal slices, score the dough with a serrated knife first, instead of cutting all the way through.) Lay the slices in the pan(s), evenly spaced, with the spirals facing up. Press down gently to spread the slices slightly. Cover with plastic wrap and set the pan(s) aside in a warm, draft-free spot for 45 to 60 minutes, until almost doubled in bulk.
Preheat the oven to 375°F. When the slices are doubled, remove the plastic and bake on the center oven rack for 30 to 35 minutes, until golden brown. As soon as the buns are done, carefully invert them onto a baking sheet, taking care not to spill hot syrup on your arms or hands. (I suggest wearing long sleeves and mitts.) Cool at least 10 minutes before serving.