On any difficulty level, play through the campaign until you reach the greenhouse in Act 2 of Chapter 2. There, destroy two waves of enemies. Around you, there also are tomato plants growing in pots. Make sure that you destroy every last one of the pots before leaving the building, precious though the plants are to their owner. A lot of the pots are positioned out in the open, on tables, but make sure that you don't miss the pot located on a high shelf. When every last pot has finally been destroyed, you'll know it because Marcus will start an extended, profanity-laced rant. When it concludes, head toward the building's exit and you should spot a new tomato plant. Investigate it to receive Dom's Toms, special saucy grenades that function like standard ones but look very different.