Heat butter in a pan, add garlic, celery and saute on low for a minute till they turn fragrant. Take care not to brown the garlic.
Add mushrooms, saute for 3-4 minutes till the mushrooms are cooked well.
Add flour & stir continuously on low for 1-2 minutes till they get mildly roasted.
Add milk, while stirring continuously and cook on low for 1-2 minutes till the milk mixes well with the flour, cooks well and turns creamy. Refer note 1
Add cream cheese, salt, pepper, italian seasoning, mix well and cook for 1-2 minutes till the mushrooms absorb the seasoning. At this stage, if the mixture gets too dry you may add 1 ½ - 2 tablespoons of milk to adjust the consistency to be creamy.
Add 1 ½ tablespoons of the creamy mushroom mix to each of the tart shells, garnish with spring onion greens and serve immediately.