Heat olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Season chicken pieces with salt and pepper, then sear 4-5 minutes per side until golden brown on the outside. Transfer to a plate and set aside—the chicken doesn't need to cook through yet.
In the same pot, add diced onion and sauté for 3 minutes until softened and fragrant. Stir in minced garlic and cook an additional 1 minute, being careful not to burn the garlic. This aromatics base builds the foundation for your crowd dinner flavor profile.
Pour in chicken broth, scraping the bottom of the pot to release flavorful browned bits. Add potatoes, carrots, celery, thyme, rosemary, and bay leaf, then return the seared chicken to the pot. Bring to a simmer and cook for 20 minutes, ensuring vegetables are tender and chicken reaches an internal temperature of 165°F.
While the cozy fall crowd chicken dinner simmers, mix cornstarch and water in a small bowl to create a slurry. Stir this mixture into the pot during the last 2 minutes of cooking to thicken the sauce into silky perfection.
Remove from heat and stir in heavy cream until fully combined. Taste the pot and adjust seasoning with additional salt and pepper as needed. Let rest for 3-4 minutes before serving so all the flavors meld beautifully together.