Preheat the oven to 400°F and line two baking sheets with parchment paper.
In a medium saucepan, bring water, butter, sugar, and salt to a boil over medium heat.
Add flour all at once, stir vigorously until a dough forms and a film coats the pan, 1–2 minutes.
Transfer dough to a bowl and let cool 5 minutes until warm, not hot.
Beat in eggs one at a time until the dough is smooth, shiny, and forms a thick ribbon.
Pipe 4–5 inch logs onto prepared sheets, spacing 2 inches apart.
Smooth tips with a damp finger and mist lightly with water.
Bake 10 minutes at 400°F, then reduce to 350°F and bake 18–22 minutes until deeply golden and puffed.
Turn off the oven, poke a small hole in each éclair, and let dry in the turned-off oven with door cracked 10 minutes, then cool on a rack.
For pastry cream, heat milk and 1/2 cup cream to steaming in a saucepan.
Whisk yolks, 1/3 cup sugar, cornstarch, and salt until pale, then slowly whisk in hot milk.
Return mixture to the saucepan, whisking until thick bubbles form, then cook 30 seconds more.
Off heat, whisk in vanilla, strain into a bowl, press film, and chill until cold.
Whip 1/2 cup cold cream with 2 tbsp powdered sugar to soft peaks and fold into chilled pastry cream.
Toss diced strawberries with lemon juice and 1 tsp sugar and let macerate 10 minutes, then drain well.
Fold drained strawberries gently into the diplomat cream to make strawberry filling.
For glaze, pour hot cream over white chocolate, let sit 2 minutes, whisk smooth, then whisk in strawberry jam.
Pierce the bottoms of éclairs with two holes and pipe in strawberry diplomat cream until filled and slightly heavy.
Dip tops of éclairs into the strawberry white chocolate glaze, letting excess drip, and set on a rack.
Garnish with thin strawberry slices and a light dusting of powdered sugar just before serving.