Heat a dutch oven to medium high. Add a splash of water and add your onions, carrots and celery. Cook for 5 minutes until the starts to brown. Continue to add water to ensure that your mixture doesn't burn. Add the garlic and cook for another minute.
Rinse the yellow split peas and add to the dutch oven and add in 4 cups of broth. Cover and simmer on low for about 20 minutes. Stir and add remaining 1 cup of broth and the thyme. Cook until the split peas breakdown and tender.