In a skillet, sauté the onions and bell peppers until they are soft, but not really cooked. 
From here you have several options.

Mix everything together in your slow cooker on low for about 6 hours.

Mix everything together in a big roasting pan and cook at about 225 in your oven for about 6 hours. Stir often.

I mixed everything into foil roasting pan. (I doubled them up because it is pretty heavy.) On my gas grill, I had two foil loaf pans filled with wood chips, one on each side above two burners on high. I put the foil roasting pan in the middle of the grill where the burners were turned off. I left it on here for about 6 hours, letting the smoke from the wood chips smoke the beans a little. Again, stir about every 30 minutes or so just to make sure the beans on the sides where the burners are don't burn. But the more often you stir, the less time the smoke stays trapped in the grill. If it gets too hot you can lower the heat under the wood chips once they start smoking. You'll figure it out.

I mixed everything into foil roasting pan. (I doubled them up because it is pretty heavy.) Put this in your smoker for about 6 hours, stirring every hour or so.