For shallots:

Combine the vinegar, wine, brown sugar, peppercorns, mustard, chile flakes, and salt in a medium saucepan. Stir over low heat until the sugar is dissolved. Add the shallots and bring to a boil. Reduce to a simmer and cook 5 minutes. Set aside to cool completely in the liquid.

Transfer the shallots and all their liquid to a jar or plastic container. Cover tightly and store in the refrigerator up to two weeks.

For tacos:

Warm the refried beans gently over low heat. Cut the butternut squash in half crosswise at the narrowest point. Cut each half in half again lengthwise. Peel each section with a vegetable peeler or a flexible sharp knife. Use a teaspoon to scrape out the seeds and strings. Cut into 1/2- inch dice. Heat large nonstick skillet over high heat. Add olive oil. When it smokes, add squash cubes and toss well. Sprinkle with 1 teaspoon salt, cinnamon, cumin and black pepper. Saute over high heat, tossing frequently, until browned outside and tender but not mushy inside, about 10 minutes.

To serve, heat tortillas quickly one by one in a very hot dry skillet, about 10 seconds per side. Assemble tacos by spooning in black beans, topping with Sauteed squash, sliced pickled shallots, cilantro sprigs, diced Serranos, and cream. Roll up and serve 2 per person.