Mix the lime juice, olive oil, cumin, granulated garlic and salt in a medium bowl. Add the chicken and toss around to coat with the marinade. Set aside for 30 minutes while you prepare the other ingredients, or cover and refrigerate up to 3 hours ahead.
Make the crema
Put the yogurt in a small bowl and crumble in the goat cheese. Stir until the mixture is smooth.
Make the guacamole
Scoop the avocado flesh into a bowl and add the lime juice and salt. Lightly mash together with potato masher or wooden spoon. Taste for seasoning and add more lime juice and salt, if you like.
Cook chicken and assemble tacos
Heat an outdoor grill to medium-high or a heavy grill pan or cast-iron skillet on the stovetop (be sure to use your exhaust fan or open a window). Oil the pan, if using. Cook the chicken, turning frequently, until browned and cooked through, about 8 minutes per side. Let the chicken rest 10 minutes, then slice or chop into thin slices or bite-size pieces.
To make the tacos, put 1 or 2 lettuce leaves on each tortilla. Put a spoonful of guacamole on the lettuce. Arrange some chicken on top then add some cucumbers, radishes, jalapeño and green onions to each one. Spoon over some of the crema and Green Sauce.