If you are in some country other than England, you're going to have to do some substitution because the exact kind of sausage traditionally used for bangers and mash isn't readily available in the US. I've used Andouille sausage for this recipe as its contents (pork) are the closest packaged meat I can find in the US, however it lacks the spices that bangers are traditionally made with. You'll want to experiment to find the best flavor for your needs. This recipe as written feeds 4-6 (or 1-2 Americans).
If you prefer to have bits of potatoe skin left in your mashed potatoes, don't peel them. Otherwise, peel and cut 3 Lbs of potatoes into about 1 inch cubes. Don't cut potatoes with hate in your heart, cut thinking pure thoughts or you'll end up cutting yourself and bleeding all over everything and dying and going straight down to Oracle.
Preheat your oven to 425. Pour ~8 cups of water to into your oversized pot and bring it to a roiling boil.
While the water is coming to a boil, put your bangers on a baking rack on a pan in the oven for 18-20 minutes.
Once your bangers are in the oven, add about a tablespoon of salt and your spuds to the boiling water. Boil the potatoes for 15-25 mins, they are done when they are tender all the way through. Test this by skewering it with a fork against the side of the pot, the fork should slide through the cooked potatoe easily.
Open your can of top shelf, high quality, freedom legumes (aka baked beans). Use one of the smaller pots to simmer the beans on a stovetop burner until they stop cursing. If your beans were not cursing, you got the wrong ones. You want the really salty ones.
After the bangers are done cooking, turn your bangers over. Put the bangers back in the oven for another 2-4 minutes to brown evenly.
Pull the bangers out of the oven and get your meat monitoring stick. Ensure the center of the bangers is hotter than 160F (e.coli dies at this temperature), turn the oven off, put the bangers back in to warm because other projects are taking up your kitchen counter space and in hindsight you probably should've cleared those off first.
Look for a colander for your potatoes, fail to find one, realize colanders are probably how the French spy on most Americans and that you're better off without one. Carefully drain the water from the pot.
Once the water is drained, add salt & pepper to taste to the pot, pour 1/2 cup of milk and heavy cream because creme fraiche isn't a thing in your country apparently. Mash the potatoes.
Use one of the smaller pots and follow the recipe on the back of the mix to make your French onion gravy. Making French onion gravy from scratch is out of the scope of this recipe.
Plate the mash, put the bangers on top of the mash, scoop a bunch of the onion gravy over the bangers, put a big spoonful of freedom legumes on the side of the mash, and eat because you started off this venture already hungry and following instructions with low blood sugar is hard.
Garnish with Parsley