By hand, or in a food processor with knife blade attached, finely chop beefsteak tomatoes; transfer to a medium bowl. Finely chop onion; add to beefsteak tomatoes. Stir in salt; let stand 5 minutes. Strain mixture through a fine sieve, pressing hard on the solids (should yield about 2 cups liquid). Discard solids.
Grate zest from 2 lemons. Squeeze enough juice from 3 lemons to equal 1/2 cup. Gently stir tomato water, lemon zest, lemon juice and remaining ingredients (except margarita salt) into cherry tomatoes. Cover and refrigerate, keeping tomatoes submerged in liquid, at least 3 days or up to 1 week.
To serve, spear each tomato with a toothpick or wooden skewer. Put some of the liquid in a glass bowl or vase; place tomatoes in the liquid. Serve margarita dipping salt on the side.