Preheat the oven to 350°. Cut a piece of parchment paper to fit the bottom of a 9 inch springform pan. Spray the pan with vegetable spray and lay the parchment on the bottom of the pan. Set aside.
Add the sugar and lemon zest to the bowl of a large food processor (at least 7-cup capacity). Pulse until the lemon is completely integrated with the sugar (about a minute) and is very fragrant.
Add the flour, baking powder and salt to the sugar. Secure the lid and rest a dish towel over the food processor (this is to prevent a puff of flour from escaping the sides (it will do that). Pulse 5-6 times until all the ingredients are well combined.
Add the diced butter to the bowl and pulse to combine (about 20 pulses). Remove one cup of the mixture and set aside for the crumb topping.
In a small bowl, combine the milk, eggs, sour cream and vanilla and whisk until blended. Make a slight well in the flour mixture and pour in the liquid ingredients. Pulse until just combined. Batter will be smooth.
Add the walnuts, oats and cinnamon to the reserved crumb mixture. Toss well to combine. Set aside.
Spread half the batter in the prepared pan, smoothing the top with a spatula. Cover the first layer of batter with the sliced berries, overlapping if you need to.
Spoon the remainder of the batter over the strawberries and smooth with a spatula. Sprinkle the crumb topping evenly over the batter. Bake for 50 minutes to one hour, or until a toothpick comes out clean.
Remove the cake from the oven and let it cool to room temperature before removing the collar. When the cake is cooled, find an edge of the parchment paper beneath the cake and gently lift it an inch or so. Slip a few fingers under the parchment paper and as you push forward under the cake, spread your hand to its widest point to support the lemon crumb cake. Quickly transfer the cake to a stand or platter.
Notes
If the batter is coming precariously close to the top of your pan, either use a slightly taller pan to compensate (maybe a springform pan) or scoop out enough batter to make one or two cupcakes. Place the cake pan on a rimmed baking sheet to catch any potential drips.