Melt butter and grapeseed oil in a stockpot over medium-high heat. Cut roast into 3 equal pieces and season with taco seasoning. Sear roast on all sides, 8 to 10 minutes.
Pour 1/2 cup salsa in the bottom of a slow cooker. Transfer roast to slow cooker and top with remaining salsa.
Cook until meat is fork-tender, 8 hours on Low or 6 hours on High. Remove from slow cooker and shred beef with 2 forks.
Set an oven rack about 6 inches from the heat source and preheat the oven’s broiler.
Toast slider buns under the broiler for 1 to 2 minutes, making sure not to burn them.
Place about 1/4 cup shredded beef onto each slider bun and top with avocados, olives, jalapenos, crema, and cheese.
Leftover beef can be used for nachos or quesadillas.
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