Purée strawberries with sweetener in a blender until smooth, then strain through a fine-mesh sieve into a large heavy saucepan. I personally don't mind the seeds so sometimes I leave them in.
Bring purée to a boil, then simmer over medium-low heat, stirring occasionally (more frequently toward end), until thick enough to mound slightly and reduced to 1 to 11/4 cups, 45 minutes to 1 hour.
Preheat oven to 200°F with rack in middle. Line a large baking sheet with nonstick liner (like a Silpat).
Pour hot purée onto liner and spread thinly and as evenly as possible into a 15- by 10-inch rectangle using spatula.
Dry purée in oven until it feels dry but is still slightly tacky, 2 to 3 hours. It shouldn't stick to your fingers.
Cool on liner on a rack until completely dry, at least 3 hours and up to 24.
Place a sheet of parchment paper over leather, then peel leather off liner and roll up in parchment. Strawberry leather keeps in a sealed bag at room temperature 1 month.
Recipe by Bistro OneSix at https://bistroonesix.com/strawberry-fruit-leather/