Combine mashed strawberries, lemon juice and sugar in a bowl and let them sit for 5-10 minutes until the sugar has dissolved and the strawberries have released their juices. If the strawberry sauce still feels gritty when you rub some of the syrup between your fingers, you can bring it to a simmer in a medium saucepan over medium heat for 5 minutes until the sugar is melted, but I haven't found this to be necessary most of the time.
1 pound strawberries, ¾ cup granulated sugar, 1 Tablespoon fresh lemon juice
Meanwhile, beat heavy cream in a large bowl with a whisk attachment until stiff peaks form. Set aside.
1½ cups heavy cream
In a separate bowl, combine mascarpone cheese, sugar, and vanilla. Beat until smooth with an electric mixer. Fold in heavy cream with a heavy spatula until evenly combined.
8 ounces mascarpone cheese, ½ cup granulated sugar, 1 teaspoon vanilla extract
Dip lady fingers into the strawberry sauce, one at a time, until soaked but not soggy. Arrange in an even layer in an 8-inch square baking dish. Spread half of the filling on top.
1 (7-ounce) package Savoiardi ladyfingers
Arrange remaining ladyfingers on top of the mascarpone cheese layer. Spread with the remaining filling or transfer the remaining filling to a piping bag fitted with a large round tip and pipe decorative dollops over the top of the tiramisu with the remaining filling instead.
Sprinkle with the grated white chocolate. I use a bar of Ghiradelli white chocolate and grate it right over the tiramisu using a zester or the smallest holes of a box grater. Refrigerate for 6 hours or overnight.
Finely grated white chocolate
Slice and decorate with additional strawberries just before serving.
Additional strawberries