Add all of the syrup ingredients (except the elderflower liqueur) to a saucepan over medium heat, stirring occasionally until it starts to boil.
Boil for 1-2 minutes until the sugar has dissolved and the liquid turns clear instead of cloudy. Turn the heat down to a gentle simmer, then let it bubble for 10 minutes to infuse the strawberry flavor. The strawberries should be soft enough after this that they can be easily mashed.
Remove from the heat and let the syrup steep with the strawberries in it until completely cool.
Strain the mixture through a fine-mesh sieve to separate the syrup from the strawberry solids. Use a spoon to press most of the liquid out of the berries.
Add the elderflower liqueur (if using) to the syrup, then bottle it and store it in the fridge until ready to use. Keep the strawberry solids too, as we'll use those for the filling. If there are any large chunks of strawberry, mash or cut them down to a smaller size before storing.
Make the Mascarpone Cream
Whip the cream just until it reaches medium peaks - this means that peaks will form but will still flop over. Don't let it go all the way to stiff peaks or it won't incorporate smoothly into the mascarpone.
Remove the mascarpone from the fridge and mix it a couple of times just to loosen it up a bit. Add it to a bowl along with the sifted powdered sugar and vanilla extract, then mix everything together just until combined and smooth - mascarpone will split very easily if it is even slightly overmixed.
Gently fold the whipped cream into the mascarpone mixture just until combined. Refrigerate until ready to assemble the tiramisu.
Assemble the Strawberry Tiramisu
Dip the ladyfingers in the strawberry syrup, one at a time, dipping them for just 1-2 seconds on each side. They will continue to absorb moisture as the dessert chills. Arrange each dipped ladyfinger into the bottom of your pan, making sure to cover as much of it as possible in an even layer. If the ladyfingers don't fit perfectly, you can just break them down to size.Fill in any little gaps with little pieces of ladyfingers as any holes will mess up the lovely defined layers.
Add a little less than half of the mascarpone cream on top of the ladyfingers layer and spread it out as smoothly as possible.
Pour the strawberry solids leftover from making the syrup on top of the mascarpone layer and spread it out to make a delicious surprise strawberry filling.
Repeat the ladyfingers step, dipping and arranging on top to cover the whole surface.
Pipe or spread the remaining mascarpone cream on top. I like to pipe it in little blobs for a classic tiramisu look - but you can spread it into little peaks with a spoon for a more rustic look.
Cover with plastic wrap and chill the strawberry tiramisu overnight. It needs at least 8-24 hours for the filling to set and all the flavors to meld together. If you try to serve it before then the slices will not come out cleanly.
Right before serving, decorate with sliced fresh strawberries. I also added some crushed up freeze-dried strawberries and some chopped pistachios.
Notes
Pan Size
This recipe was developed for an 8-inch square dish. You can make it in other shapes and sizes, but the thickness of the layers will vary. If you want to use a 9x13-inch dish, you can double the recipe.
Ingredient Notes:
Strawberries - You can use fresh or frozen strawberries. If using frozen there is no need to let them thaw first, just add them straight to the pan with the other ingredients.
Elderflower Liqueur - Optional but highly recommended. It adds a floral complexity and makes the strawberries shine. I used St Germain, but any will work (or use elderflower cordial for a non-alcoholic version).
Mascarpone Cheese - Use full-fat mascarpone. If your brand has excess liquid in it, be sure to drain that off first.