Heat oven to 300°F and arrange a rack in the bottom of the stove with plenty of room above it for the covered pot to fit. 

Generously season chuck roast on all sides with salt and pepper. 

Heat 1/4 cup olive in a large Dutch oven with a lid over medium-high heat. Once oil is shimmering add roast and brown on all sides, about 10-12 minutes. 

Remove roast to a plate and add onion, tomatillos, garlic, jalapeños, and cumin. Season with salt and pepper, and cook, stirring occasionally, until browned, about 6-7 minutes. Add broth and scrape up any browned bits at the bottom of the pan. 

Return roast to the pan, cover and transfer to the oven. Braise until meat is very tender, about 3 hours. Transfer roast to a plate to cool. Save liquid and vegetables in the pan. 

Once roast is cool enough to handle, shred meat into bite-sized pieces. Return to cooking liquid. Add zucchini and cilantro and stir to combine. 

Warm tortillas in a dry pan or comal. Transfer to a tortilla warmer or wrap tightly in a clean kitchen towel to keep warm. 

Take one tortilla at a time and place 3 tablespoons of filling on one edge. Roll tortilla tightly around the filling and secure closed with a toothpick. Repeat with remaining tortillas and filling making sure to keep tortillas warm as you go. 

Heat 2 cups of vegetable oil in a large frying pan over medium-high heat. Once oil is hot (test by dipping a tortilla edge into the oil, it should bubble immediately), add flautas, toothpick-side down, a few at a time. 

Cook about 2 minutes per side, or until crisp and golden. Transfer to a paper towel-lined baking pan. Repeat with remaining flautas. Enjoy immediately, or let cool, cover, and refrigerate. Flautas can be eaten cold, at room temperature, or warmed in a 350°F oven for 15-20 minutes.