Roast the potatoes: Preheat the oven to 425°F and line a baking sheet with parchment paper. Spread the potatoes and half of the diced onion out on the baking sheet in a single layer. Sprinkle evenly with the pepper, then drizzle with 2 tablespoons of vegetable oil. Stir to coat the onions and potatoes evenly, then bake until golden-brown, about 45 minutes, stirring halfway through cooking for even browning.
When the potatoes have finished cooking, sprinkle with the salt and place back in the oven. Turn the oven off-the residual heat will keep the potatoes warm while you prepare the rest of your ingredients.
While the potatoes are cooking, in a medium (2-quart) saucepan, heat the olive oil over medium heat. Add the remaining half of the diced onions and the diced jalapeño and sauté until onions are translucent, about 3 minutes.
Add the cumin and sauté for another minute, until aromatic. Add the pinto beans, cover, and reduce heat to low. Let simmer on the back of the stove for 30 minutes, then turn off heat and let sit, covered, until ready to serve.
Heat remaining vegetable oil in a medium nonstick skillet over low heat. Break the eggs into a bowl and whisk until whites and yolks are thoroughly blended. Pour into the pan, still on low heat. Cook until the eggs are set but still a bit wet, stirring often with a wooden spoon. Remove from heat and set aside.
Place the tortillas on a plate and cover with a damp tea towel, or use a microwaveable tortilla warmer. Heat the tortillas until steaming-hot and pliable, about 30 seconds. Wrap the warmed tortillas in a cloth napkin or clean tea towel and place in a bread basket, or place on a tortilla warmer so they stay flexible and warm while you assemble the tacos.
Place two warm tortillas side by side on each plate. Using a slotted spoon to reduce excess liquid, spoon a quarter-cup each of the scrambled eggs, pinto beans and potatoes. Sprinkle each tortilla with 2 tablespoons of cheese. Top with a heaping tablespoon of tomatoes and top with any optional garnishes. Serve immediately.