For the green tomato chowchow relish: Combine the vinegar, sugar, salt, celery seeds and mustard seeds in a small saucepan, bring to a boil and cook until the sugar is dissolved. Remove and let the mixture cool completely.
Combine the tomatoes, red and yellow bell peppers, jalapenos and onions in a medium bowl. Add the cooled vinegar mixture, olive oil and black pepper and mix until combined. Cover and refrigerate for at least 1 hour and up to 24 hours before serving.
For the BBQ remoulade: Whisk together the mayonnaise, mustard, barbecue sauce, cornichons and green onions in a medium bowl. Season with salt and pepper, cover and refrigerate for at least 1 hour to allow the flavors to meld.
Heat the grill to high for direct grilling. Grill the sausages until golden brown and charred on all sides and just cooked through, about 10 minutes.
To serve, slice the sausages and arrange on a platter. Serve with bowls of the BBQ remoulade and chowchow relish, with grilled bread on the side.