Heat vegetable oil in a small saucepan.

Add Sichuan peppercorns to the hot oil and cook over moderate heat until darkened (about 4 minutes).

Strain the oil, reserving the infused oil.

Heat 2 tablespoons of the strained Sichuan peppercorn oil in the saucepan.

Add dried red chiles, thinly sliced garlic cloves, and thinly sliced ginger and cook, stirring, over moderate heat until the chiles darken (about 3 minutes).

Add rinsed and chopped fermented black beans and cook for 30 seconds.

Add chicken stock and boil over high heat until reduced by half (about 3 minutes).

Season the sauce lightly with salt.

In a large skillet, heat the remaining strained Sichuan peppercorn oil until shimmering.

Season the trimmed beef tenderloin slices with salt.

Add half of the seasoned meat in a single layer to the hot skillet and cook over high heat until browned and medium-rare (about 1 minute per side).

Transfer the cooked meat to a platter and repeat with the remaining meat.

Reheat the black bean sauce and add the scallions, cut into 2-inch lengths.

Pour the sauce over the cooked meat, scatter the cilantro leaves on top, and serve immediately.