In a medium saucepan, stir the strawberries and sugar together. Let the mixture stand until the fruit's juices release, about 30 minutes. If you're using frozen strawberries, heat them with the sugar over medium heat until they're defrosted, about 2 minutes.
In a small bowl, whisk together the cornstarch and lemon juice. Combine them with the strawberry mixture. Simmer the sauce on medium-low until it thickens, about 10 minutes, then set it aside.
Place the milk, eggs, flour, sugar and salt, plus 3 tablespoons of the butter, in a blender and process until smooth, scraping the sides as needed, about 1 minute.
Heat the oven to 175° and warm a 10-inch nonstick skillet over medium heat. Brush the skillet with some of the remaining butter, then form a crepe in the pan by quickly pouring 1/4 cup batter into pan. Then lift the pan from the heat and tilt it in a circular motion to evenly coat the bottom surface. Cook the crepe until the edges pull away from the skillet and the bottom is golden brown, about 2 minutes. Flip the crepe with a rubber spatula and continue cooking until the crepe is lightly browned on the opposite side, about 1 minute more. Transfer the crepe to a baking sheet and place it in the oven to keep warm. Repeat with remaining batter.
To assemble the crepes, warm the strawberry filling over low heat, if desired. Add 2 tablespoons of the filling to the center of each crepe and roll it up. Scatter strawberries over it, then dust it with confectioners' sugar, if desired.