Cut out the core of the tomato (where it attached to the plant), then cut in half widthwise. Squeeze each half gently to remove the seeds; discard. Cut tomato into 1/4-inch dice and scoop into a bowl. Remove the chiles from the can, then scoop the 2 tablespoons canning sauce into the bowl with the tomatoes. Slit open the chiles and scrape out the seeds. Chop chiles into small pieces and add to the bowl, along with the red onion, green onion, shrimp, herbs and crème fraiche or Mexican crema. Mix gently but thoroughly, then
taste and season with salt (usually about 1 generous teaspoon).
If using the squash blossoms, pull off the little green sepals around the bottom of each flower, then carefully break off each flower from its base, dislodging the petals (they’ll stay together as a single unit) from the bulbous bottom; the pointy pistils inside should stay attached to the base. Cut the flower petals crosswise into thin strips.
Just before serving, pile a portion of the shrimp mixture onto each of the chips, chalupitas or tostadas and arrange on a serving platter. Sprinkle with the cheese and then the optional squash blossoms and serve right away.