In a small saucepan or pot, combine the peanut butter with the hoisin sauce. As both are soft, they should combine very easily. A whisk may work best to mix it together.
Once the peanut butter and the hoisin sauce has fully combined, turn the heat on medium and cook the mixture until it starts to bubble slightly.
Once the mixture starts to heat up, add water in small amounts. The amount you add will vary on how thick or thin you want your sauce. I personally like a thinner sauce so I will eyeball the water and pour enough to thin out the sauce. Usually for this amount it will be around ¼ cups and if I want it thinner I will add a touch more. Keep in mind that the sauce will also thicken once it cools down.
Once you reach your desired consistency and everything is well combined, turn off the heat and taste your sauce. Add in the lime juice & sesame oil until well combined.
Chill sauce or serve immediately.