Preparation of Baking Environment: Preheat the oven to 400 degrees Fahrenheit. Simultaneously, place an empty baking sheet on the center rack to heat.
Pie Crust Setup: Insert the premade pie crust into a suitable pie pan and transfer it to the refrigerator to keep it chilled.
Butter Browning Process: In a small saucepan, melt the butter over medium heat. Continue heating until the butter turns golden brown, emitting a nutty aroma and showing brown flecks amidst the foam. This should take approximately 5 minutes. Remove from heat and allow to cool.
Filling Mixture Preparation: In the bowl of a stand mixer equipped with a whisk attachment, beat the eggs vigorously for 2 minutes until they become light and airy. Gradually incorporate the granulated and brown sugars along with the corn syrup, mixing continuously. Following this, integrate the cooled butter into the mixture. Subsequently, add the milk, vanilla extract, salt, lemon juice, and flour, ensuring each ingredient is thoroughly combined. Gently fold in the pecans and toffee bits, then pour the resulting mixture into the chilled pie crust.
Initial Baking: Place the pie on the preheated baking sheet in the oven. Immediately reduce the oven temperature to 350 degrees Fahrenheit. Bake for 30 minutes.
Streusel Topping Preparation: While the pie bakes, prepare the streusel topping. In a medium bowl, mix flour and brown sugar. Incorporate the cold butter by cutting it into the flour mixture using a pastry blender or two knives, until the mixture resembles coarse sand. Keep this mixture refrigerated or frozen until ready to use.
Adding the Streusel and Final Baking: After the initial baking, carefully remove the pie from the oven, noting the filling will still be quite fluid. Evenly distribute the streusel topping over the pie. To prevent over-browning, cover the edges of the pie with a pie shield or a custom-made aluminum foil shield. Return the pie to the oven and bake for an additional 30 to 40 minutes, until the center is just set but still slightly wobbly.
Cooling: Remove the pie from the oven and place it on a cooling rack. Allow the pie to cool completely before serving to ensure the filling sets properly. For optimal results, prepare the pie a day in advance of serving.