Any kind of medium to large-sized type of green tomato you please, as long as it’s an underripe tomato and not a green when ripe tomato. An underripe tomato is one that hasn’t ripened (turned red) yet, are green in color, are very hard, and has a tart flavor. While you won’t want to bite into an underripe tomato, their tart flavor is reminiscent of that of tomatillos and is what makes this salsa so fantastic. Think of this recipe as a green tomato salsa verde.