Heat oil in a deep pan over medium-high heat and sear the beef cubes until well browned on all sides.
Sprinkle mixed spice over the meat, add cardamom pods, and stir briefly.
Press the garlic into the pan and cook 30 seconds until fragrant.
Pour in hot water gradually, scraping the bottom, and simmer until the liquid takes on a deep color.
Whisk yogurt in a bowl, then slowly mix in ladlefuls of the hot broth to temper it.
Turn off the heat completely, pour the tempered yogurt over the meat, and stir until the sauce is smooth and glossy.
In a separate pot melt butter with oil, add the washed rice and crushed mastic, and toss to coat every grain.
Add water to cover the rice by about 1 cm, bring to a boil, cover, and steam on low until fluffy.
Fluff the rice, plate it beside the creamy beef, and garnish with lemon slices.