For the chili lime butter: In a large bowl, stir together the softened butter, parsley, shallots, miso, Fresno chiles and lime zest and season to taste with salt and pepper. In a small pot, gently melt about 1/2 cup of the chili lime butter over the grill. Set aside the remaining chili lime butter.
For the pork: In a small bowl, combine the salt, chili powder, garlic powder, paprika and pepper. Drizzle the tenderloin with olive oil, then season all over with the spice rub. Place in the refrigerator to marinate for 1 hour.
Preheat half of a grill to medium-high heat.
Grill the tenderloin on the hot side of the grill on all sides until nicely browned, 3 to 4 minutes per side. Once seared, move the tenderloin to the cool side of the grill and let cook with the grill lid down, turning occasionally, until the pork reaches 140 degrees F internal temperature. Remove the pork from the grill and let rest for 10 minutes.
For the peaches: While the pork rests, grill the peaches. Drizzle the halved peaches with olive oil and season with salt and pepper. Place flesh-side down on the hot part of the grill and cook until nicely charred, 2 to 3 minutes. Flip the peaches and spoon some of the reserved chili lime butter into the well of each peach. Cook another 2 to 3 minutes on second side, and then remove to a serving platter.
For the salad: In a large bowl, whisk together the lime juice, olive oil and Dijon and season with salt and pepper. Add in the watercress, cilantro and parsley leaves and toss together.
To serve, slice the rested tenderloin into 1-inch medallions, and then drizzle with the melted butter before serving alongside the peaches and watercress salad.
Any unused chili lime butter can be stored in the refrigerator up to one week, or frozen up to 2 months.