1. In a small bowl combine all the spices. If you are missing one or two of the ones that only call for 1/4 or 1/8 of a teaspoon, don't sweat it. 
2. Place the beef on a cutting board and pat dry with a paper towel. Use a sharp serrated knife to cut the beef into 2 or more chunks (the more cuts you make, the faster it will cook.) 
3. Rub the spices all over the beef chunks. Use all of it!
4. Turn your Instant Pot on to the "Sauté" setting. When it is nice and hot, add 2 tablespoons olive oil (or any oil will do). Add half of the beef to the hot oil (don't crown the pan, or your meat will not sear properly.) Sear for about 1-2 minutes, until well browned. Use tongs to flip the meat over. Sear the other side for 1-2 minutes and remove to a plate. 
5. Continue searing the other half of the meat.
6. When all the meat is browned, turn off the sauté  setting. Add all the meat to the pot, as well as HALF of the sliced red onion (save the other half to garnish the meat after it's cooked), 1 tablespoon garlic, and 1/2 cup beef broth. Don't add the lemon juice right now.
7. Put the lid on the pressure cooker and make sure the valve is set to "Seal." 
8. Pressure cook on high for 90 to 110 minutes. If you have a smaller roast (or cut into several chunks) choose 90 minutes. If you have a large roast and only cut it in half, choose 110 minutes.
9. Let the pressure cooker release naturally for 10-15 minutes (that means don't do anything for 10 minutes when it beeps). 
10. Use a long spoon to carefully move the valve over to "Vent." Lots of angry steam will come out the top.
11. Open the lid and take out a forkful of meat. Taste it and see if it is as tender as you want it to be. If it's still chewy, replace the lid, bring it back up to pressure and cook another 10 minutes. 
12. When the meat is done, remove from the pressure cooker and shred. 
13. If you prefer to have the meat crispy on the edges, (this step is optional) spread the shredded meat out onto a baking sheet and drizzle with some of the cooking juices. Broil on high, with the rack close to the flame, for about 5-10 minutes, checking every 2 minutes to flip the meat and make sure it's not on fire. (Hey, broilers are no joke.)
14. Squeeze lemon juice all over the shredded beef. 
15. Serve the beef with Tzatziki Sauce (not optional!)
16. You can present it burrito-bowl style with Tzatziki Sauce, Cauliflower Rice, (or regular rice) cherry tomatoes, sliced red onions,  sliced cucumbers, and fresh herbs. OR you can serve these as gyros, which would be all of the above (minus the cauliflower rice) wrapped up in a warm pita. 

Slow Cooker Instructions:

1. Prepare the meat as described, but there is no need to chop into smaller chunks. Sear the meat with the oil in a pan on the stove, using medium high heat. (Same times as above)
2. Add the meat, red onion, garlic, and beef broth to the slow cooker. Cook on low for about 8 hours until fork tender.