For the pepper relish: Preheat the oven to 425 degrees F. Toss the bell peppers with the oil and place on a foil-lined baking sheet cut-side down. Roast until the skin blisters and pulls away from the flesh, about 30 minutes. Transfer to a bowl and cover with plastic wrap to steam to make it easier to remove the skin, about 10 minutes. When cool enough to handle, peel the charred skin off the peppers and chop into 1/4-inch pieces.
Combine the vinegars, sugar, salt and saffron, if using, in a small saucepan. Heat over medium heat to infuse the saffron into the vinegar and melt the sugar, 5 minutes. Stir in the peppers.
For the beef: Turn the oven down to 325 degrees F. Toast the panko breadcrumbs in a small dry skillet, stirring occasionally, until golden brown, about 5 minutes. Mix together the salt and black pepper, and then sprinkle all over the beef. Heat the oil in a medium skillet over medium-high heat and then add the beef and cook to brown on all sides, about 5 minutes. Remove and let cool enough to handle. Brush the beef with the Dijon, and then pat with the panko breadcrumbs. Transfer to a rack set over a baking sheet or roasting pan and roast until a thermometer reads 125 to 130 degrees F, about 35 minutes. Transfer to a cutting board to rest. Slice the tenderloin 1/4-inch thick.
For the kale: Meanwhile, whisk together the olive oil, lemon juice and mustard in a medium bowl. Add the kale and season with salt and black pepper.
To assemble, spoon 2 to 3 tablespoons of the pepper relish on the bottom of each roll. Divide the meat among the sandwiches and top with the kale and remaining bread slices.
The pepper relish can be kept in the refrigerator for 5 to 7 days. Make ahead of time, or reserve leftovers for use in other recipes.