Heat the coconut oil in a large, heavy pot or Dutch oven over medium heat. Add the onion and sea salt and cook for 5 minutes, stirring a few times.
Raise the heat to medium-high, add the beef strips in one layer and cook for 2 to 3 minutes, until the colour of the meat changes and some strips brown.
Add the cremini mushrooms and cook for 5 to 6 minutes, until the liquid they release has mostly evaporated.
Stir in the garlic, tomato paste, Dijon mustard and black pepper and cook for 1 minute.
Pour in the beef stock and add the bay leaf and dried porcini mushrooms, scraping the bottom of the pot.
Bring to a gentle simmer, reduce the heat to low and cover tightly. Simmer for 45 to 55 minutes, stirring every 15 minutes and adding a splash of water as needed to keep the meat at least half covered.
Meanwhile, boil the cubed potatoes in salted water for 15 to 20 minutes until tender, then drain and keep warm.
Taste a strip of beef. It should be tender with no rubbery chew. If it is still firm, cover and simmer another 10 to 15 minutes.
Scoop a few tablespoons of the hot cooking liquid into a cup, dissolve the tapioca flour in it, then stir it back into the pot. Simmer uncovered for 1 to 2 minutes until glossy.
Remove the pot from the heat, discard the bay leaf and stir in the sour cream until smooth.
Stir in the peas, cover and stand for 3 to 4 minutes. Stir in the parsley and serve over the boiled potatoes.