In a large bowl, mix together the crust/topping ingredients: butter, brown sugar, oats, flour, salt, and baking powder. Mix until there are no butter chunks and the mixture is starting to clump together.
Press 3/4 of the mixture onto the bottom of a lightly greased 7x11-inch pan. (TIP: Line pan with parchment paper sticking out of the sides to easily lift the dessert out of the pan to cut into bars after baking.)
In a separate bowl, mix together the sweetened condensed milk, lemon zest, lemon juice, and egg yolk. Spread onto the crust.
Toss raspberries with the flour and granulated sugar. Mash lightly with a fork, keeping some fruit intact. Then dollop the raspberries into the lemon filling.
Crumble the remaining crust mixture over the top of the lemon raspberry filling.
Bake for 30 to 35 minutes or until the crust browns and the center of the filling is set. (Check with a toothpick.) Cover with foil if browning occurs before filling is set.
Chill for 1-2 hours before cutting and serving. Keep leftovers covered and refrigerated.