A sweet spot between simple salmon and comforting potpie, this dish is packed with iron and Omega 3 fatty acids. The rich, crisp puff pastry contrasts with the spring vegetable and fish filling: light, creamy, and nourishing.
Preheat oven to 375 degrees. Place half of spinach in a saucepan over medium heat. Cook, stirring and adding remaining spinach as it wilts, until all is just barely wilted; drain. When cool enough to handle, squeeze out excess liquid; coarsely chop. (You should have about 1 scant cup.)
Unfold dough onto a parchment-lined baking sheet; freeze 10 minutes. Turn a 9 1/2-inch deep-dish pie dish over on dough; cut a circle, using dish as a template. Cut out a 1 1/2-inch hole in center. Cut scraps into 1/2-inch-wide strips to decorate top, if desired. Freeze dough 10 minutes. Brush with egg. Arrange strips on top as desired and bake until puffed and golden, about 30 minutes.
Meanwhile, season fish with salt; place in pie dish in an even layer. Melt butter in a saucepan over medium-high heat. Add scallions, season with salt and pepper, and cook until soft, about 2 minutes.
Add flour; cook until it has a slightly nutty aroma, about 1 minute. Whisk in stock, milk, and 1/4 cup water. Add potatoes; bring to a boil, then reduce heat to medium and simmer, covered, until just tender, 6 to 7 minutes. Stir in peas; return to a boil. Stir in spinach, tarragon, and lemon juice; remove from heat. Season with salt and pepper.
Pour mixture over fish in pie dish. Top with blind-baked crust; bake until bubbling around edges, 20 to 22 minutes. Let cool 20 minutes before serving.