I do keto yearly to cut weight. Makes portions just very easy for me. For some quick fast food options in a pinch, McDonald's triple with cheese* sans bun (easy to order on mobile), extra pickles/lettuce. Chipotle/Moe's/Qdoba lots of options. I like a double meat salad with fajita veg, cheese, sour cream, salsa of choice, no rice/bean/corn salsa. Wings/salad at lots of pizza joints, verify they're not fried. Panera salads, just check the dressings. Greek restaurants usually have a nice selection of salad or grilled meat and veg options just nix the rice or bread.. and sadly hummus.

Another reliable fast food option is Jimmy John's. Their "un-which" is your sub of choice wrapped in lettuce leaves. The gargantuan un-which is my go-to.