1. Place raisins in small bowl. Add boiling water to cover; let stand 5 minutes. Drain and pat dry on paper towels. Set aside.
2. Dissolve yeast in warm water in bowl; let stand 5 minutes. Add milk, egg, and vanilla. Combine 3-1/2 cups flour, the sugar, and salt in food processor; pulse to blend. Add butter and lemon peel; process until mixture resembles fine crumbs. With machine on, add yeast mixture through feed tube and process until it just holds together, 15 to 20 seconds. Transfer dough to floured surface. Add raisins to dough and knead until smooth and elastic, adding additional flour if necessary, about 10 minutes. Transfer to greased bowl, turning dough to grease top. Cover and let rise in warm, draft-free place until doubled in bulk (about 2 hours).Make Streusel::
3. Meanwhile, combine oats, brown sugar, and cinnamon in food processor; pulse until mixed. Add butter and walnuts; pulse just until crumbly.
4. Grease two 9-inch round cake pans. Punch down dough. Divide in half and spread in prepared pans. Sprinkle with streusel, cover, and refrigerate overnight.
5. Remove pans from refrigerator and let stand at room temperature 20 minutes. Meanwhile, heat oven to 350 degree F. Bake coffee cakes 30 to 35 minutes or until tops are golden. Cool on wire racks. Makes 2 cakes, 6 slices each.