On the opposite end of the stem on the peaches make a crosscut in both directions so there is a “+” sign. Drop in boiling water for 30 seconds to a minute depending on the firmness of the peaches. If hard to the touch boil for a minute minimum. Remove peaches from boiling water with slotted spoon and when cool enough to handle peel the skin back from the cut. Discard the skin and pit. Rough chop the peaches and purée in a food processor or blender. Set aside.
In a medium saucepan add the rinsed raspberries, sugar and juice from the lemon. Cook on medium heat and using a spoon stir and break down the fruit. In a not the small bowl mix together the cornstarch and water and as you are stirring the raspberry mixture pour in the cornstarch slurry. Continue to stir as this thickens. Remove from heat and set aside to cool. When cool press the fruit through a sieve to extract the sauce and discard the seeds. Set aside.
In a heated sauté pan add the oil and chopped onions. Sauté until translucent and slightly browned.
In your food processor or blender add the sautéed onions, raspberry sauce, peach purée, chipotle peppers, kosher salt, vinegar and tomato paste. Purée until smooth. Season to taste with salt and for a more tangy flavor adjust with more vinegar if needed. Allow to cool and store in cleaned, airtight jars and refrigerate. Good for 7 days chilled.
Use as a basting and BBQ sauce on any grilled meat, or as a dipping sauce.