Toss the coleslaw mix, chopped jalapeños, and brine in a large bowl. Season with salt and pepper and set aside so the vegetables can soften slightly in the acid while you cook.

Whisk the sour cream and lime juice together in a small bowl until smooth; the consistency should be thin enough to drizzle easily.

Adjust an oven rack to the middle position and heat the oven to 400 degrees. Place the refried beans in an oven-safe bowl and arrange the tostadas in a single layer on a baking sheet; heat until both are warmed through, about 10 minutes.

Heat the vegetable oil in a large skillet over medium-high heat until it shimmers. Add the onions and bell peppers and cook until the vegetables are softened and starting to brown at the edges, 8 to 10 minutes.

Stir in the garlic and cook until fragrant, then season the vegetable mixture with salt and pepper to taste.

Spread a layer of warm beans over each tostada shell, then top with the sautéed vegetables, the jalapeño slaw, and crumbled cheese. Drizzle with the lime crema and garnish with cilantro, serving immediately while the shells are still crisp.