Remove outer leaves, stalk, and core from cauliflower. Divide into large florets. Rinse in a colander then pat dry with a paper towel. Slice each floret into 1/4-inch-thick slices. Place in a large bowl.
Chop onion. Melt butter over medium-low heat in a large skillet and add onion. Cook and stir 2 to 3 minutes, or until onions are soft.
Mince garlic. Add to onions, along with water (or broth), tomato paste, thyme, coriander, allspice, salt, pepper, and red pepper flakes. Stir well, then turn heat to low. Cover and cook until cauliflower is crisp-tender, about 3 to 5 minutes.
Chop parsley. Right before serving, add parsley and toss. Squeeze juice from half a lemon over the cauliflower. Garnish with extra parsley and wedges from the remaining lemon if desired.