Heat your cast iron skillet (or largest skillet) over medium heat for 5 minutes. Season the flank steak with kosher salt + pepper.
In a measuring cup, combine the beef broth, tapioca flour, worcestershire + dijon mustard. Whisk to combine really well and remove any lumps from the tapioca flour. Set aside.
When the pan is hot, add the avocado oil. Lay the steak in an even layer and brown for 2 minutes per side. Don't move the steak at all during the 2 minutes so it can really sear and get crusty, golden brown. If your skillet isn't big enough, sear in batches so you don't overcrowd the pan. Transfer the cooked beef to a plate.
In the same skillet, add the mushrooms and onions. Season with a pinch of kosher salt + pepper. Cook for 8-10 minutes, stirring occasionally, until golden brown. Scrape up any brown bits on the bottom of the pan from the beef.
Add in the garlic, and cook for 30 seconds until fragrant.
Give the beef broth mixture another whisk to redistribute the tapioca flour. Then pour over the beef broth mixture over the mushroom mixture, stirring constantly until it comes to a simmer. Once it simmers, it will activate the tapioca starch and the mixture will thicken.
Add in the cooked beef and the hearts of palm pasta. Simmer for 1-2 minutes to heat the pasta and beef.
Add in the sour cream and stir to combine. Taste for salt + pepper.
Serve warm. You can top with finely chopped parsley or fresh thyme leaves.
Leftovers will keep in an airtight container refrigerated for at least 5 days.