Preheat oven to 400 degrees.
MUFFINS: Beat together the egg, milk, lemon zest and oil in a medium bowl.
Mix together in a separate bowl the baking powder, white sugar, 1.5 cups sifted flour and 1/2 t. salt. Stir into the wet ingredients just until the dry ingredients are moist, about 8 rotations. Add raspberries; stir about 2-3 more times.
Fill muffin cups about 2/3 full.
STREUSEL TOPPING: Stir together the remaining flour, brown sugar, cinnamon and oatmeal. Cut in with a pastry blender the butter until the mixture resembles coarse crumbs.
Sprinkle topping over each muffin.
Bake 20-25 minutes. Remove from pan immediately. Makes about 12.
NOTE: The key to making a truly tender muffin is in NOT overmixing your batter. Stir it as little as possible. It's OK if the batter is somewhat unevenly mixed; it "corrects" itself in baking. Sifting the dry ingredients lends to the light texture, so don't neglect this step, either. Use either a traditional sifter, or a mesh strainer to sift your ingredients.
Try a Microplane grater for your lemon zest, or a Messermeister zester/channel tool. I find these to be the easiest way to get fresh zest from citrus. Use only the colorful exterior of the lemon-- the white peel part adds only bitterness.