In a medium saucepan, heat strawberries, sugar and lemon over medium heat for 10 minutes. The strawberries should be reduced to a juicy, bubbly jam.
Either transfer to a heat proof bowl and use an immersion blender, or transfer to a food processor to blend into a smoother mixture. It doesn't need to be perfectly smooth - but if you'd prefer it smoother, just push it through a fine mesh sieve to remove the seeds.
Place in the fridge to cool while you prep the mousse.
In a medium bowl, combine mascarpone cheese and 1 tablespoon sugar, and use a hand mixer or stand mixer to beat on high speed until combined.
In another medium bowl, combine heavy cream, ½ tablespoon sugar, vanilla and salt and use a hand mixer to beat on high speed until it forms soft peaks.
Fold the heavy cream mixture and lemon juice into the mascarpone mixture until completely combined.
Spoon mousse mixture into a pastry bag to pipe into serving dishes.
Dividing between two serving dishes (or 4, if you'd like smaller portions): Start with a a spoonful of strawberry puree, then a layer of mousse, and repeat twice more until you've used all the ingredients.
Top with diced strawberries and/or fresh lemon zest and serve. Enjoy!
Note about mousse texture: Your lemon mousse may not be completely smooth, but it should still be creamy, light and fluffy. Some people have noted that they thought the mascarpone "curdled" when they folded in the lemon juice, but that should not be the case (unless you went a little crazy with your whisk or spatula). Take a peek at the photos as well as my original mascarpone whipped cream recipe - similar consistency! I also have an entire post dedicated to making lemon mascarpone whipped cream for more detail and video.