Whisk together the water with ingredients from all 3 of the envelopes. Pour about 1/4 cup of the liquid on the bottom of the crockpot, place the roast on top. Don't be tempted to season the roast. There is plenty of salt and other seasonings in these envelopes.
Pour the remaining liquid mixture all over the top and all around the roast. Pour the jar of picante sauce on top of the roast, cover and cook on low for 8 hours, or on high 4 to 5 hours.
To thicken gravy, remove the roast and tent with foil to keep warm. Carefully transfer the contents of the crockpot to a saucepan and stir in a slurry of one tablespoon of cornstarch with just enough water to loosen. Stir into the sauce, bring up to a boil, reduce and simmer until thickened.
To Make Sliders: Preheat oven to 400 degrees F. Butter the insides of dinner rolls and toast, butter side down, on a baking sheet in the oven just until warmed through and browned on the bottoms. Meanwhile in a skillet, caramelize a sliced onion in a tablespoon of canola oil and one tablespoon of butter. While onions are browning, use the lowest setting on the microwave to gently warm the leftover beef, just enough to take the chill off. Use a fork to shred the meat. Place a scoop of beef on the bottom half of dinner rolls, then top with caramelized onion, and some finely shredded Swiss cheese. If you like, stack all the sliders back on the baking sheet and cover the whole pan tightly with aluminum foil. Return to the oven just long enough to melt the cheese. Stack the sliders on a platter and serve with condiments at the table or with a nice sauce like a Sour Cream Horseradish Sauce.