Adapted from Food to Love. I decided to add mushrooms and zucchini, and you could add them to the slow cooker during the last 30-45 (high) or 90-120 (low) minutes of cooking, but I chose to quickly sauté them up in a little oil in a nonstick skillet with a sprinkle of salt while I was making rice for the meal. Basil is the preferred herbal garnish for this dish, but I find it much easier to get nice cilantro in the winter.
Heat the coconut oil over medium heat in a cooktop-safe slow cooker insert or a separate skillet and add the beef chuck. Brown on each side, 5-8 minutes per side, and then remove to cut into 2-3" pieces.
Add the curry paste to the remaining fat in the pan. Stir and cook until the oil starts to separate. Add the curry leaves and the sliced onions with a tiny pinch of salt. Cook for 5 minutes and then add the garlic and ginger. Stir and cook another 2 minutes.
Add the makrut lime leaves and the coconut milk and stir until the ingredients are incorporated with one another.
If you are using a separate pan, everything will be added and stirred in the slow cooker insert now. Add the chicken stock and 2 tablespoons each of fish sauce and palm sugar. Add the beef back into the slow cooker insert, along with any accumulated juices.
Place the slow cooker insert onto the base. Cook for either 4 hours on slow or 6-8 hours on high.
Before serving, taste for more fish sauce or sugar. Stir in the sautéed zucchini and mushrooms. Serve garnished with either cilantro or basil, chopped cashews and sliced cucumbers. John and I liked it drizzled with Sriracha sauce as well. Serve with Jasmine rice.