In a large skillet, warm 2 tablespoons olive oil over medium heat. Once the oil is shimmering, add the chopped onion and ¼ teaspoon salt. Cook, stirring often, until the onion is tender and translucent, about 4 to 5 minutes. Add the garlic and cook, stirring constantly, until fragrant, about 30 seconds.
To make your enchiladas, preheat the oven to 400 degrees Fahrenheit with racks in the middle position and upper third. Lightly grease a 13 by 9-inch pan.
Bring the mixture to a simmer, then reduce heat as necessary to maintain a gentle simmer, whisking often, for about five minutes, until the sauce has thickened a bit and a spoon encounters some resistance as you stir it. (The sauce will further thicken as it cools.) Remove from heat, then whisk in the vinegar and season to taste with a generous amount of freshly ground black pepper. Set aside.
Before you make the sauce, measure the flour and spices into a small bowl and place your tomato paste near the stove. In a medium-sized pot over medium heat, warm the oil until shimmering (it should be warm enough that the spices sizzle on contact). Pour in the flour and spice mixture. While stirring constantly with a whisk, cook until fragrant and slightly deepened in color, about 1 minute. Whisk the tomato paste into the mixture, then slowly pour in the broth while whisking constantly to remove any lumps.