Preheat the oven to 350 degrees. Heat the extra virgin olive oil from Spain in a medium skillet over a medium heat. Add the onion and cook until translucent, 3 minutes. Add the broccoli and cook, stirring frequently, until the broccoli begins to soften, 3 minutes. Stir in the salt, pepper and mustard powder. Allow to cool completely.
In a large bowl whisk together the milk and flour until the flour is dissolved. Add the eggs and whisk to incorporate.
Pour the filling into the pie crust and bake until a knife inserted into the middle comes out clean, 50-65 minutes. Allow to cool for 5 minutes before slicing.