How to prepare chicken tartare?

As with all raw-meat or sushi dishes, this will never be 100% safe to eat; at best we are talking about a calculated food safety risk, with steps taken to mitigate that risk when possible. For this reason, it is CRUCIAL that the chicken be kept at fridge temperature as much as possible. When cutting, use a pre-chilled cutting board if possible, and serve immediately. For extra safety, blanch the chicken to kill surface pathogens, and slice off the cooked exterior.

Given the slimy, gelatinous texture of raw chicken, I believe the best results would come from finely dicing the chicken (small cubes) or very thinly slicing it. These approaches seem to work well for sushi and tuna tartare, which have similar textures. As with sushi, you need a shaving-sharp knife to get clean cuts.

For flavorings, your goals are twofold: add flavor and kill pathogens. An acidic sauce or marinade is obligatory, to reduce the pH below 4 and render it hostile to pathogens. The acidity should also reduce the slight sulfurous notes in raw chicken, by reacting with the sulfur in the molecules and making them less volatile. Taken to an extreme, this could produce a result much like ceviche, where the acid denatures the proteins and produces a texture akin to cooking. Fresh cut, finely-minced garlic is delicious and also hostile to pathogens. Many aromatic herbs also have antimicrobial properties, with fenugreek being a notable standout in this area.

There are a couple ways you could develop this into a full dish from there:

• Do it up like a ceviche: fresh lemon/lime juice, garlic, chili flakes, onions, salt, and maybe a little cilantro or coriander.
• Go European: white wine vinegar, thyme, rosemary, parsley flakes, garlic, black pepper, salt, and maybe a little olive oil
• Go sushi! Marinate it in rice wine vinegar, soy sauce, and spices, and serve it in sushi rolls, accompanied with a LOT of wasabi and pickled ginger. Those condiments help mitigate the food safety risk with their antimicrobial/antiparasite properties.
• Go sushi part 2: marinate it as above, but serve alone with a horseradish-based sauce containing lemon. EDIT: Apparently there is a real recipe along these lines. Toriwasa, or chicken sashimi. I found several recipes using wasabi (horseradish), soy sauce, parsley, and sake. The chicken's exterior is cooked, but the interior remains raw.

I should add a couple caveats to this: I have serious doubts about raw chicken as an ingredient, even with the best cooking possible. At best I think it will be edible but not delicious. The problem is that raw chicken is incredibly bland, and where it isn't bland, it has an unpleasant, slightly sulfurous flavor. The texture is somewhat off-putting too. I might be wrong, of course, but I think this is better handled as a theoretical exercise and not an actual dish. If you try any of these ideas and get food poisoning, it's your own dang fault for eating raw chicken... you boob.