Combine beans, half the garlic, half the minced onion, and salt in a 4-6 qt slow cooker with 3 1/2 cups water.
Puree remaining garlic, ginger, Serrano chiles, turmeric, and paprika with 2 tablespoons water in food processor; set paste aside.
Heat oil in a 10 inch skillet over medium-high heat. Add chile de arbol, coriander, cumin, and cinnamon; cook until fragrant, about 1 minute. Add remaining minced onion; cook until browned, about 1 minute.
Add paste; cook for 2 minutes. Stir in tomatoes; cook until liquid has mostly evaporated, about 12 minutes.
Add tomato mixture, cilantro, garam masala, lemon juice, salt, and pepper to beans in slow cooker. Mix well and check level of the water. Add more water, if necessary, so that there is at least 1 inch above level of the bean mixture. Cook on low for 7-8 hours and on high for 4-5 hours.
Serve over rice and garnish with sliced red onion.