In a 3.5-qt cast iron braiser, heat the butter over medium-low heat. Add the leeks and scallions and cook until softened, 8 to 10 minutes (no color on this).
Meanwhile, in a large liquid measuring cup, whisk together the chicken broth, extra bouillon paste (if using), and the Dijon mustard.
Add the garlic to the pan and cook, stirring constantly, until fragrant, about 30 seconds. Add the flour and cook, stirring constantly, 1 minute.
Stir in the chicken broth mixture, salt, pepper, garlic powder, and onion powder.
Add the spinach bring to a simmer over medium heat. Simmer until the spinach is wilted, about 2 minutes.
Reduce the heat to low. Stir in the heavy cream and herbs. Then, stir in the English peas and chicken. Remove from heat and let cool while the oven preheats.
Stir in the spinach and cook until wilted, 7 minutes.
Preheat the oven to 400 F (205 C).
Working with one sheet at a time, scrunch and arrange the phyllo pastry over the filling.
Combine the 2 tablespoons (28g) melted butter and olive oil. Brush this onto the pastry.
Bake until the pastry is golden and the filling is bubbling, 30 minutes. Let cool 10 minutes before serving.