Scrub potatoes and dry roast on baking sheet at 400° until they are just tender, about 30 minutes. Cool slightly.
Place potatoes on a work surface and press with the back of a spatula or heavy skillet to about 5/8-inch thickness. Hold at room temperature until service or cover and refrigerate until ready to use.
Stir together BBQ sauce (or BBQ Spice Blend) and butter. Refrigerate until ready to use.
BBQ Butter Spice Blend
Blend all ingredients in a processor until thoroughly combined. Makes about 4 cups. Store in airtight container if not using immediately.
To make BBQ Butter stir together 2 cups seasoning blend with 2 cups softened unsalted butter. Keep refrigerated. Use in place of prepared BBQ sauce and melted butter, adding 2-3 tablespoons BBQ butter per 6-ounce portion of potatoes.
Per serving: Scale recipe according to desired servings
Using a heavy skillet sized to fit potatoes in a single layer, lightly coat with olive oil. Place over medium-high heat; brown 6 ounces smashed potatoes on both sides, about 5 minutes.
Add 1/4 cup onion and about 6 strips sliced jalapeño. Continue cooking 2-3 minutes to lightly cook onion and jalapeño.
Remove pan from heat and quickly stir in 3 tablespoons BBQ butter. Transfer to serving plate. Garnish with pickled peppers.