Arrange the diced rye bread in an even layer on a baking sheet. Place the baking sheet in a preheated 375°F oven.
Remove from the oven and stir to encourage even browning. Return to the oven until all the moisture is removed and the bread begins to brown, about 3 more minutes.
SOUP
Render the bacon. Drain the grease into a soup pot. Hold the bacon for later use.
Heat the grease in the soup pot. Sauté the onions and garlic. Add the butter.
Reserve 8 oz of the romaine. Add the remaining romaine to the pot and sauté 2–3 minutes.
Add the flour and cook until it is light brown.
Add the broth and bring to a simmer. Add the cream, milk, half the tomatoes, and half the reserved bacon. Simmer for 5–6 minutes.
Whip in ⅓ cup mayonnaise. Simmer 2 more minutes, then remove from stove.
ASSEMBLY
Finish the soup with salt, pepper, and butter. Pour into bowls. Garnish soup bowls with the remaining romaine, croutons, reserved bacon, remaining tomato, 4 Tbsp mayonnaise, and sage.