Add thinly sliced onions to the bottom of a 6 quart slow cooker and set aside.
In a large mixing bowl, add chicken thighs, garlic, brown sugar, paprika, oregano, salt, pepper, peri peri sauce and lemon juice. Using gloved hands, mix ingredients well, coating all sides of the chicken.
Place chicken, in a single layer (as best as you can, tuck the chicken tightly together) on top of the onions in the bottom of the slow cooker. Cover and heat on high for 3 hours or low for 5 hours or until it shreds easily with 2 forks. (low is preferred for the best texture)
When cooking is complete, use 2 forks to shred the chicken (or can leave whole if preferred) and serve on top of a bed of rice.