Place your peeled, rinsed, and halved potatoes into a pot and add just enough cold water to cover the potatoes. Add a pinch of salt, and bring to a boil. Once boiling, bring the heat to low, cover the pot, and let the potatoes simmer for 15 minutes or until tender enough to poke through with a fork. In the meantime, work on your white sauce.
Melt butter in a large saucepan over medium-high heat. Add the garlic and onion, and cook for 2 minutes or until the onions are translucent and fragrant. Add carrot, celery, and fresh herbs, and simmer for 5 minutes, or until carrots and celery are soft, stirring occasionally.
Bring the heat down to medium-low and stir in the flour until the vegetables are coated and the mixture is thick and sticky. Then, add half of the milk, stirring constantly as the mixture thickens. Continue to stir as you add the rest of the milk. Bring the heat up to medium and keep stirring until the mixture is thick and creamy. This should take about 2 minutes.
Remove from heat and add a pinch of salt and pepper. Taste, and season with more salt and pepper as you see fit.
Drain all of your canned fish; place fish fillets onto a cutting board and roughly chop it all. Place the fish in a medium 8x8 or 8x10 (ish) baking dish. Toss and mix the fish around to ensure the smoked fish and mackerel are evenly dispersed.
Top the fish with the frozen peas, and then the white sauce.
Once the mashed potatoes are done, drain the pot. Place the pot back onto the burner over medium heat and heat just enough to shake out any extra water (optional). Add the butter, a pinch of salt and pepper, and a splash of the milk. Mash with a potato masher and add more milk as needed until you get a smooth consistency.
Spoon the mashed potatoes over the fish and white sauce and spread them out evenly. Drag a fork across the top of the potatoes to create lines or zigzags—creating these ridges will create crispy bits.
Finally, sprinkle the grated Pecorino Romano over the potatoes and bake for 15 minutes.
To get more crisp on the top, once 15 minutes is up, turn the broiler on and broil the pie until you get golden crispy bits on top. (Time varies with each broiler, so just keep a very close eye on it).
Remove from oven and let it stand for five minutes to cool.
Garnish with fresh parsley if you want, and serve. (I like to eat mine with some horseradish.)
Please note that it's best to spread the mashed potatoes while they're hot.