Heat a large, high-sided skillet (or a wide-bottomed pot), over medium high heat.
Salt and pepper the beef chunks.
Add about a tablespoon of oil to the pan and swirl. It should start to shimmer if the pan is hot enough.
Add 1/3 of the seasoned meat to the pan. Place the pieces of meat so that they are not touching each other (You don't want them to steam--see photos). Turn the pieces with tongs to brown all sides. Remove to a plate and repeat 2 more batches, adding more oil as necessary. You should only be browning meat for 5-10 minutes total. (You only want to brown the outside, not cook the inside.)
Remove the final batch of meat, then add the onions to the pan, adding more oil first if there is none left.
Saute the onions over medium heat for 5-8 minutes, or until mostly cooked through. Do your best to scrape up the browned bits.
Return the meat to the pan. Add the water, tomato sauce, garlic, salt, beef bullion, pepper, and cumin.
Bring the mixture to a boil, then reduce to a low simmer. You want there to be some movement. If the mixture is completely still, turn it up a little. I had my burner set just below medium.
Simmer for 2-3 hours, or until the meat is tender and the gravy has thickened. Stir occasionally.
To serve authentically, put the meat, some rice, cheese, and guacamole in a warm flour tortilla.
It's important to note that this is not a shredded beef recipe; "Carne Guisada" literally means "Stewed Beef" so you want it to be just like beef stew, but without any veggies.