Heat the oil in a large skillet over medium high heat. Add the onions and cook until soft, about 25 minutes. Add garlic and marjoram and stir until fragrant and garlic softens, about 2-3 minutes. Add in the tomato paste and stir constantly for about 3 minutes. Add paprika and cayenne pepper and stir well. Remove from heat and let flavors meld together.
Add the mixture to the heated crockpot and add vinegar. Stir well, cover, and let vinegar absorb into vegetables for about 2-3 minutes. Add in chicken stock, bay leaves, and lemon zest.
Sprinkle sirloin chunks with salt and pepper. In same skillet for onions, add beef chunks (you can wipe the skillet if you want to, but I think it adds to the beef so I didn't). Brown chunks on all sides and then add to crockpot. Deglaze skillet with red wine, scraping the sucs from the pan. Add wine to crockpot as well. Stir mixture, cover, and let cook on high for at least 3 hours. The longer it sits, the more tender the meat will be.
Uncover crockpot and carefully take out beef with a slotted spoon and transfer to bowl. Tent with aluminum foil. Pour crockpot liquid and onions into a large sauce pan and bring to a boil until reduced to about 5 cups, about 15 minutes. Let cool momentarily and blend with immersion blender until mixture is nice and smooth. Season with salt and pepper. If you want to thicken the liquid just a tad, mix 1 tablespoon cornstarch with 1 tablespoon cold water in a small bowl. Add to liquid and stir. Then add blended liquid, along with beef chunks, back into crockpot. Turn to low or warm setting. Let simmer for about 5-10 minutes and serve along with rice or egg noodles and some roasted vegetables.