Preheat the oven to 450 degrees F (230 degrees C). Line a baking sheet with parchment paper.
Toss lemon slices and pork tenderloins with 4 teaspoons oil in a large bowl. Place pork on the prepared baking sheet, surrounded with lemon slices. Season pork with salt and pepper.
Roast in the preheated oven until slightly pink in the center, about 30 minutes. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C).
While pork is roasting, bring a large pot of lightly salted water to a boil. Cook orzo in the boiling water, stirring occasionally until tender yet firm to the bite, about 11 minutes.
Heat 1 tablespoon oil in a large saucepan over medium heat. Add garlic and cook until fragrant, about 2 minutes. Add chard; cover, and cook until tender, about 7 minutes.
Add cooked orzo and roasted lemon slices to the chard mixture; toss to combine. Serve with the pork.