Step 1Heat the oven to 375 degrees and generously butter 6 (one-half cup) straight-sided ramekins or individual souffle molds. Warm the olive oil in a skillet and add the garlic. Cook until the garlic turns translucent, about 3 minutes, then add the mustard greens and cook, stirring, until they are quite soft, about 5 minutes. Cool slightly.
Step 2Combine the greens mixture, the ricotta, egg yolks and salt in a food processor and puree until smooth.
Step 3Beat the egg whites until frothy. Add the cream of tartar and continue beating until well-defined, stiff peaks form and retain their shape. Do not over-beat.
Step 4Gently fold about one-half cup of the greens puree into the egg whites, using a hand-held balloon whisk or a spatula. Cut straight down through the puree and the egg whites, scrape the bottom and lift the egg whites over the puree. Turn the bowl a quarter turn and repeat. Keep doing this until the puree is fully incorporated, 3 or 4 times.
Step 5Add the remaining puree and fold it in the same way. When you're done, the puree should be evenly distributed but there may be small patches of egg white remaining. Do not over-mix.
Step 6Using a large soup spoon, divide the souffle mixture among the ramekins. If you have excess batter, bake it in another ramekin or discard it. (The recipe can be made to this point, covered tightly with plastic wrap and refrigerated for up to 2 hours or frozen for up to a month. Souffles can be baked straight from the refrigerator. If frozen, remove the plastic wrap immediately and warm at room temperature for 30 minutes before baking.)
Step 7Put a jellyroll pan in the heated oven and arrange the souffle ramekins on it. Bake until the tops are puffed and golden brown in spots, 18 to 23 minutes. A sure sign of doneness is when the souffle becomes extremely fragrant. Do not over-bake, or the center will be dry. Remove and serve immediately.